No, this feat isn't impossible. A WoW-player named Watola has managed to level 100 characters up to level 100, which has a nice symmetry to it. Watola started work on this project towards the end of Mists, working with two accounts (each account is limited to 50 characters, so this is a must) to level two characters at once with Recruit-a-Friend bonuses. Always leveling with one tank meant getting into dungeons and clearing them, even solo, was easy XP. Getting to 85 in this way took Watola 5-8 hours, depending on the class -- likely taking advantage of rested XP (easy to gain when leveling so many alts) and heirloom bonuses.
At 85, an alt would be paired with a max-level hunter to farm Pandaria elites, which, if they were tagged by the alt before being killed by the hunter, the alt would get full experience for. This would earn a level every 20 or 30 minutes with full rested, and Watola would cycle through rested alts to maximize XP gain. At 90, Watola took his army of alts to Timeless Isle, gearing them up to an average ilvl of 507 to be ready for questing in Warlords.
On Draenor, Watola leveraged garrison missions that awarded player XP and got clearing each game zone down to a science, minimizing time spent out questing on each alt. Presumably with rested XP, each zone broke down like this:
- Frostfire Ridge - 1 hour and 6 minutes
- Shadowmoon Valley - 44 minutes
- Gorgrond - 42 minutes
- Talador - 40 minutes
- Spires of Arak - 39 minutes
- Nagrand - 30 minutes