See the end of Act 18 for your quest log. We have JUST arrived in Zangarmarsh at the start of level 62, however, and have a ton of stuff to do here and a little still left from Hellfire.
Here's the map:

Scene 1: The Dead Mire (level 62)
Our goal is the northeast corner of the zone. Along the way, take your time to collect any Fulgor Spores you see and kill any Marshfang Rippers. Do not make this a priority since we'll be passing in and out a couple times.
In the southwest corner of the Dead Mire, there is a spore bat named Sporewing. Find him and kill him. No hurry on this either.
The real objective is to find a patch of soil somewhere in the Dead Mire. There are quite a few of them. Just find one and return to Telredor. Save Fertile Spores and Bog Lord Tendrils if you can afford the space. (As well as Unidentified Plant Parts.)
You'll get a follow-up from The Dead Mire. Go follow it up by heading back out there and killing Withered Giants, BUT as soon as you get a "Withered Basidium" to drop, immediately go back to Telredor. You'll get one new quest to do out here.
Make a third and final trip to The Dead Mire and finish both quests you've got out here. If you haven't finished Fulgor Spores or Menacing Marshfangs by now, then go hunt the area around Telredor for them.
Quests completed: Unfinished Business, The Dead Mire, Withered Basidium, Fulgor Spores, Menacing Marshfangs, An Unnatural Drought, Withered Flesh
Scene 2: Umbrafen Environs (level 62)
Travel south to Umbrafen Lake. Kill eels and hydras. Eventually make your way to the southwest corner of the lake and "investigate" the large machine there.
From there, head east to Umbrafen Village and fight your way to the top of the large building at the south end of the village. Kill Kataru and then clear the central of the three hut clusters to reach Kayra Longmane and escort her back to the Refuge.
Turn around and go back for two things – to finish The Umbrafen Tribe if you haven't, and to look for the fen strider Boglash. Boglash will NOT be an easy fight solo, so hopefully you can muster some help, but it is possible to kill him solo at 62 with most classes. Just make sure you have a healing potion.
Return to Cenarion Refuge, perform the follow-up quest "As the Crow Flies" by immediately using the item you are given, and then back to Telredor.
Quests completed: Too Many Mouths to Feed, Umbrafen Eel Filets, Disturbance at Umbrafen Lake, The Umbrafen Tribe, The Dying Balance, As the Crow Flies
Scene 3: Funggor Caverns (level 62)
Return to Cenarion Refuge. Hopefully you noted that when you finished The Umbrafen Tribe, three quests opened up from separate people. If you don't have those, go grab 'em now. (They are A Damp Dark Place, Safeguarding the Watchers, and Saving the Sporloks.)
Head south, and feel free to kill fireflies along the way, until you get to the edge of the zone. West of the road into Terokkar is a cave on your map, with a narrow path that leads up to it. Enter the cave and start battling the large thorn elemental guys. You'll notice there are also little mushroom people, but they're actually friendly.
The cave is a large circle with some stuff in the center. Go 180 degrees around the cave and locate Ikeyen's Belongings on a wide flat rock. From there, face the "middle" of the cave and work your way in toward Lord Klaq. I have no idea why this quest is marked as Group. It was easy to solo. The mobs next to Klaq are not linked, so single-pulling one of them makes the fight trivial.
Fight your way back out, and ride north to that huge machine in Umbrafen Lake that we "discovered" before. Clear up to it, and use your packet of ironvine seeds on it. Beware the naga areas, they can be very dangerous overall.
Do one more thing that is COMPLETELY unrelated to anything else, but we want to take care of. (Remember how many times you went in and out of the Dead Mire?) Follow the zone wall west, past The Lagoon, past Feralfen Village, to the Ruins of Boha'mu. Walk up the steps and into the temple in the back, and you will get the objective complete message. Hearth out.
Quests completed: A Damp Dark Place, Safeguarding the Watchers, Saving the Sporloks, The Ruins of Boha'mu
Scene 4: Feralfen Village (level 62)
Part two of the quest-chain-of-many-trips returns to Feralfen Village. We have one other thing we can do in that area, though, so ride west down the road toward the Twin Spire Ruins, and then look south of the road for Blacksting, a blood red firefly. He hangs out between the road and Feralfen Village. Kill him, and then explore the village for 6 idols.
But just so this trip doesn't feel completely useless, take a little field trip to the northwest around Serpent Lake and north to the Orebor Harborage. Turn in the quest to go there, grab the flight point, and head back to Telredor.
Quests completed: Blacksting's Bane, Idols of the Feralfen, Orebor Harborage
Scene 5: Bloodscales and Darkcrests (level 62)
Look for a partner. You aren't doing this next one alone. While you look for a partner, go poach some spore bats and fen striders. (And fireflies too, if you haven't yet finished Diaphanous Wings.)
Partner in tow, head to the Bloodscale Grounds to the east of Serpent Lake and the Darkcrest Enclave east of the Lagoon. Each has a leader that needs to be killed, who is elite and unmanageable by one person, and each has another one of those machines that you need to use your seeds on.
After you kill the steampump overseers, you may get an item called "Drain Schematics" that starts a quest. Swim to the very center of Serpent Lake and dive into Coilfang Reservoir to complete the objective for that quest.
Once you're all set, go finish your spore bats and fen striders.
Quests completed: Leader of the Bloodscale, Leader of the Darkcrest, A Warm Welcome, Diaphanous Wings, Gathering the Reagents, Drain Schematics
Scene 6: Back to Hellfire (level 62/63)
Following the completion of Drain Schematics, you will have a mission that sends you back to Cenarion Post in Hellfire. So fight your way through Thornfang Hill to return to that annoying orange wasteland and report in.
From there, head south into the Ruins of Sha'naar and collect relics and kill taskmasters along the way. But your focus will be to talk to the elemental masters, who send you to Naladu in the back.
Sneak around Arzeth and into the rear left area where there is a hut where Naladu resides. Speak with him, and then tiptoe into the hut straight behind the steps Arzeth patrols, and return to Naladu. Talk to each of the three elemental masters in their respective areas. On the final return to Naladu, he will provide you with the means to defeat Arzeth. Use the staff on him, and he will no longer be elite, and an easy kill. Hopefully by the time you complete all this, you're able to finish your other two quests here.
Start seeking help for Colossal Menace. While you are looking for a couple partners, kill boars and bring their blood samples back to Cenarion Post. Take the resulting mixture to the first boar you find, pour it on, and then kill it and go back.
If you haven't found help for Colossal Menace yet, write it off as something to come back to later. The quest requires you to kill 5 the colossi, which should result in acquiring a Crimson Crystal Shard that leads to killing another colossus in the camp, and then a final difficult quest back in Fallen Sky Ridge. Take your group to the northwest corner of the huge red crystal and use your Seed of Revitalization to spawn Goliathon. Kill him and go back to the post. (Goliathon is NOT an easy kill.)
With all that complete, and a quick trip to Temple of Telhamat, you can return to Zangarmarsh. Your Hellfire quests should be completely cleaned out at this point.
Quests completed: Keep Thornfang Hill Clear!, Sha'naar Relics, Naladu, A Traitor Among Us, The Dreghood Elders, Arzeth's Demise, Demonic Contamination, Testing the Antidote, Colossal Menace, Crimson Crystal Clue, The Earthbinder, Natural Remedies, The Seer's Relic
Scene 7: Sporeggar (level 63)
Head to the west end of the zone. Along the way stop off at The Ruins of Boha'mu to finish the last part of the quest series there. (Just enter the temple, right-click the potion, and talk to the elder.) At "Cenarion Watchpost", which is just before the western exit into Nagrand, you will find Watcher Leesa'Oh by herself. Talk to her, and then travel west toward the Spawning Glen. On the outskirts of the area, you should find Fasshn, who needs your help on behalf of Sporeggar. Sporeggar is its own faction, with its own quest hub, and in order to do those quests, you will need to go through Fasshn.
Head into the Spawning Glen, pick up 10 Mature Spore Sacs, and then come back to him, and then to Leesa'oh. You'll probably return to the Spawning Glen for more sacs later, but you'll get some rep during our next few quests, so don't rush it. (In fact, if you've done the Dead Mire quests and saved your bog lord tendrils, you're probably into neutral already. Collect glowcaps you see, but don't make a big fuss over those either since there's a good chance you will completely pass over the reputation level where you can turn them in – Blizzard, can we get an extension on these through friendly at least?)
Leesa'Oh will send you into Quagg Ridge just to the east. Same enemies you saw in the Spawning Glen, but this area has some very weird issues with the respawn on patrols, so be careful. They only go through the center of the area, so stick to the walls. Collect 10 ground spawns, and report back. When you leave, sneak out the northeast exit and look for a huge nether ray named "Count" Ungula, and kill him.
Head northwest between the two water areas into Sporeggar and say hi to the natives. Once you are on good standing with them, they will offer a number of quests to go into Coilfang Reservoir, so that will be next on your agenda, but one more task to finish things up first.
North of Sporeggar is Bloodscale Enclave. Hunt through the area for your new friends, and also to reach the fourth and final naga machine for Balance Must Be Restored. Return to Sporeggar and you'll find that it's a repeatable quest, but once is plenty. Return to town.
Quests completed: Messenger to the Feral'Fen, Watcher Leesa'Oh, Natural Enemies/More Bog Tendrils, The Sporelings' Plight, Observing the Sporelings, A Question of Gluttony, The Count of the Marshes, Sporeggar, Glowcap Mushrooms/More Glowcaps, Now That We're Friends, Balance Must Be Restored.
Scene 8: The Underbog and The Slave Pens (level 63)
As before, it's important to actually run instances now. You'll want to have those quests done to further advance your leveling without using all the high-end quests so you have those for cash later. You're also going to want to grind as much reputation as you can rather than quest for it, so the sooner you can get to honored or revered with a faction, the better.
Also some of the quest rewards for these new Sporeggar quests are rather nice. The Everlasting Underspore Frond is a wonderful item for leveling. So are the Infused Mushrooms.
You'll find a fourth quest when you swim down into Coilfang, where the splits to the different instances are. Slave Pens is on the far left, and Underbog is on the far right. The other left tunnel leads to Steamvaults, which is a bit out of our league at the moment.
And, as before, you do not have to do this now. There's nothing really hinging on you completing these instance quests or upgrading your gear or anything else. But whenever you can do this is a good time to do it, which is why it's being brought up now that the quests are in your log.
Quests completed: Oh Its On, Bring Me A Shrubbery!, Stalk the Stalker, Lost in Action
Scene 9: The Fen of Daggers (level 63)
The Hewn Bog is east of Orebor Harborage, and north of Bloodscale Enclave. Head there and start killing ogres. By the time you've killed the standard ogres and the shamans you need for Ango'rosh Encroachment, you should have just a portion of the mushroom samples you need for Familiar Fungi. That's alright. Fight your way across the bog to Daggerfen Village in the northwest. Ideally you'll have at least 8 samples by the time you get to the village.
Daggerfen Village has two creature types, and one is a stealther, so be careful. The other puts a poison on you that will eventually stun you. Fight into the center cluster of huts, where the one large central building is, and look around on the tables in the area for a poison vial. Then enter the large building and climb the stairs. Stop just before the top floor to recover, and then carefully enter the attic to where Chief Mummaki and his two guards are, along with two other separate mobs behind you. If you're careful, you can handle the chief and his guards, although by all means take any help in the area you can get. Then clear the other mobs and pick up the book in the corner.
Finish killing muckdwellers for Daggerfen Deviance, and then fight your way back across the bog. You still don't need to completely finish, but try to have most of the mushroom samples by now. Head into Orebor to turn in. This could make you neutral to the Kurenai, unlocking a few new quests.
Quests completed: Ango'rosh Encroachment, Daggerfen Deviance, Secrets of the Daggerfen, Wanted: Chief Mummaki
Scene 10: The Island of Ogres (level 63/64)
Return to the Hewn Bog to finish your mushroom samples, and then ride south to Watcher Leesa'oh. Note that two of the new quests ask you to kill Marshfang Slicers and Marshlight Bleeders. So knock those off any chance you get. In fact, after killing the slicers you need nearby, ride down the west edge of the zone in the hopes of finding the purple fireflies. (The Fenglow Stingers also drop the venom.) You will, however, make one stop along the way.
There are several islands in Marshlight Lake. The southernmost of these is covered with bogstroks rather than naga. Find and kill Terrorclaw on that island.
You might also kill the sporebats along your trip north, if you haven't finished Fertile Spores yet. However, when you get to the Ango'rosh Grounds, feel free to avoid the ogres there and just continue to the northwest corner where a fallen mushroom connects Ango'rosh Stronghold to the mainland. Cross over, and kill ogres. Loot boxes, and head UP the island (rather than in the cave) toward Overlord Gorefist. Kill him, and then return to Orebor.
Quests completed: Familiar Fungi, The Terror of Marshlight Lake, Lines of Communication, Overlord Gorefist, Stealing Back the Mushrooms, Fertile Spores
Scene 11: Three Heads Aren't Better Than One (level 64)
Zangarmarsh has almost been drained of quests, so turn in any loose ends in the zone you have, and then head toward the drain in the center of Serpent Lake and start killing hydras. Return to Maktu in Orebor when you finish.
Dive back into the lake, this time staying closer to the western island, looking for Mragesh. Kill him, and report back.
Quests completed: Natural Armor, Maktu's Revenge