Island Expeditions:[5] 3-Player co-op with optional 3v3 gameplay. Players will explore uncharted isles across the Great Sea. There is azerite and other unclaimed resources bubbling to the surface that is highly sought by both factions. Complete varied and dynamic objectives with a large variety of challenges to overcome. Race against advanced AI opponents designed to thwart players using tactical and strategic moves that consistently change. Optionally, players can choose to race against other players, creating a 3v3 PvP environment. 

Before anyone asks - yes there are archived copies of old addon versions both on our github repository and on several of the addon sites we use, and these will continue to be available after Cataclysm comes out, so you'll be able to get those old files if you really want them. I'm not going to bother making these more easily accessible unless you give me a very good reason. "I'm on a private server and I need them since we didn't upgrade to Cataclysm" is a BAD reason - and it's illegal.
To give you some perspective I leveled both a LF Ret Paladin and BE Rogue (so one started from level 20 and the other from level 1). I focused on getting the achievements I was missing for the Loremaster, so not the most efficient route by any means. I was using full enchanted heirlooms (minus the ring) and the WoW-Pro addon though, as well as having War Mode turned on all the way.
Most of the time you spend leveling will involve questing and slaying monsters, but there’s also a lot of travel. Travel earns you little experience (you do gain a bit for discovering new areas), so it’s wise to keep travel to a minimum. The new level scaling system, which scales zones to your level within a preset range, helps with that. You can choose what zones you want to experience and stick with them until you finish their quests.
These guides are completely automated and work in-game, so you can see step-by-step instructions as you play. The guides automatically move to a new step as you complete the quests. You are told where to go i.e. the questgivers for accepting and turning in quests along with where and how to complete the quests. Zygor has more steps and details, which would be more suited to beginners. Dugi condenses the info into fewer steps, but still has all the necessary questing info.
These stat priorities help to get a rough idea of what gear to go for, but the most important thing to get as soon as possible is your stat weights. Stat weights help you zero in on which stat is the most effective at increasing your healing. However, healers have no grand simulator, as there are a lot more variables that can affect your overall performance than there are for DPS specs. In light of that, healers use fight analysis tools like WoWAnalyzer and the Teachings of the Red Crane spreadsheet that takes logs from WarcraftLogs and gives you accurate weights that would improve your output.
The way Zygor's works is there is an addon called ZygorGuideViewer that is just a UI addition that displays whatever guide you pick (which doesn't have to exclusively be Zygor's Guides). Granted, it offers some other cool capabilities like auto accepting & turning in quests and selling greys, but these are features that other addons can do as well. ZygorGuideViewer works within the bounds of LUA, which makes the addon itself (and therefore its use) "legal".
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.