Blizzard have announced that the invites for today's stress test have all gone out, with a reminder there are more tests coming, as well as clarified how exactly the general invite process works. Also, unfortunately for everyone that doesn't have one yet, they're also sending out fewer and fewer beta invites, as they're nearing their goals for current testing.
The vanilla WoW Horde leveling guide you see on this site was originally made back in 2006 by Joana (AKA Mancow, or FuriousPaul). The guides have been tweaked many times over the years to make things "faster". The leveling guides were made because of all the messages I got from people asking me how I was able to level up so quickly on new realms. I was first to level 60 on 7 different realms (3 with Mancow and 4 with Joana), even winning Blizzard's "First to Level 50" contest they held back in 2006.