This is a really cool old study that looks at how people switch to a run in order save energy - at a certain speed (which differs among individuals) it is less costly to run than to walk. We switch to a run because running uses less energy than walking would.

Forcing someone to walk when they’d rather run, or run when they’d rather walk, burns more calories. (And causes injury, but hey, calories burned, baby!)

I remember learning about this applied to various animals, too, and how this plays into the idea of humans being efficient at catching critters to eat because we can jog along for ages and wear them out. We just don’t stop, and eventually the prey drops with exhaustion. BBQ time! Humans: 1, Antelope: 0.