The Horde 12-20 (Barrens + Stonetalon Mountains) guide has been rewritten and revamped.  In addition, there has been numerous tweaks to the speedrun route to make things faster and easier to follow.  Also, in case you haven't noticed I am now adding about 40% more info per step and redoing all the images.  Unlike with my previous vanilla guides, I am now giving a brief explanation of where to go and what to do with each step along with any other helpful tips.

Some dungeons are also much better than other. Some take a long time to finish and some are quick. Some have few quests and others have many. The ideal dungeons have a lot of quests and are quick to finish. If you are sent to a long dungeon that you have already completed, you can leave but you won’t be able to enter a new dungeon in 30 minutes. Dungeons like the Stockade are super quick, so it will still be good XP if you are sent here more than once.
Level 120 comes and goes. Island Expeditions have unlocked, but it turns out they’re not much fun, and the most rewarding versions are only accessible to premade groups. You turn to Warfronts only to realize they’re not available this week. Fine. You can always grind out better gear. Except as you do, you notice your new gear has higher Azerite requirements than your old gear, which means your shiny new items have fewer unlocked traits than what they replaced. Discouraged, you decide to roll as one of the cool new Allied Races. Except you can’t because they’re all locked behind reputation grinds.
I have tried it a few times. It's good if you're new to the game. Otherwise not so much. I found I leveled slower by using it, as it's suggesting to take the longest possible route usually. Basically following roads everytime, when you could take an easy shortcut. Especially if the shortcut involves a bit of fall damage, it's out of question. The "go here" spots don't also update if you take a shortcut, and then the waypoint arrow points to wrong direction. But perhaps it has been updated since then.
jame, addon version or your guide is F$%&ING AWESOME, THANK YOU. there is a part in the first section of the guide where im instructed to take shindrell's not to kayneth stillwind, but there is no instruction to pick up that quest on the way through astranaar, if i find anything else like this ill let you know, love your guide, thanks for taking the time to put it together!!
I do not recommend going for TBC, although I remember Hellfire peninsula to be decent, everything else is rather slow. Instead go for WotLK, Borean Thundra gets you levelled very fast, and then you can head straight for Icecrown at 67, which is also insanely fast, although admittedly has a few elite quests. You can go to Sholazar Basin alternatively. 2 zones should get you 60-80 no problem.