Perhaps a silly question, but can creatures that don't require sleep (such as Outsiders, Constructs and Undead) hustle without suffering nonlethal damage since they've no "sleep cycle"? For that matter, can any activity tire them?
A character can hustle for 1 hour without a problem. Hustling for a second hour in between sleep cycles deals 1 point of nonlethal damage, and each additional hour deals twice the damage taken during the previous hour of hustling. A character who takes any nonlethal damage from hustling becomes fatigued.
Only creatures immune to nonlethal damage can hustle indefinitely. This is normally creatures that have no con score, and any creature that is specifically immune to nonlethal damage. Even creatures that don't sleep normally need rest.
Most constructs and undead fall into that category though, so yeah.. an army of undead or constructs could get places pretty quick.
It's the reason Eberron's Warforged were so devastating: an army that didn't need to eat, sleep or breath. Generals would have to come up with completely new tactics for these things. Suddenly, there's no supply lines to be cut, and that lake you thought was protecting your back has an army walking across the bottom. Yikes!