Come to think of it, our legs (including associate muscles like the gluteus) are indeed disproportionately large for our overall body size.
It would be interesting to know how do we measure up against other mammals in terms of... uh... I forgot the technical name, it's the ability to absorb and metabolize oxygen. It's determined by lung size among other things, and it's a strong predictor of endurance.
Add to that a large brain, capable of planning, and driven by purpose and by an understanding of the concept of time in general and future in particular.
Large legs, oxygen metabolism, brain capable of long-term planning - well, that's the perfect trifecta that could produce an excellent endurance runner.