15h? You can do it in 7h reliably if you preplan and know your route.
Leveling really isnt part of the core game anymore. In part because classes are designed to be ”less than” untill they get the borrowed power from current expansions level cap.
And just as with a raid tier, anything that isnt current patch, is completely trivialized by the current patch. Which leads to leveling being the same thing.
Also, with blizzard having added the leveling boost, they have thrown in the towel of any leveling experience that isnt from Expansion level cap to the next.