It's also important to keep in mind that the D&D movement rules- all wonkiness aside- don't assume you go the entire "overland travel" distance at one stretch. The rules specifically state that the creature doing the moving is assumed to be spreading this travel out over a full day of 16 hours or so, with plenty of rest breaks and meal breaks. Putting aside the notion of whether I or anyone else really have a move rate of 50, the fact is that going that fast for a full 8 hours is next to impossible for anybody except a serious marathon runner, just as genshou implied.