I'm a closet Science Friday listener. Science Friday is a program on NPR every Friday (no surprise) that discusses a wide range of scientific topics.
The topic today fascinated me because they were discussing the amazing human body. According to the scientist they were interviewing, the human body can run longer and farther than any other animal (birds not included). The scientist said that if you were to have a marathoner run a marathon against a horse, dog, whatever, that the marathoner would win.
At the speed a marathoner runs, a horse and other mammals are required to gallop, or whatever their equivalent of a run is. Because of the way their physiology works, they are unable to cool their bodies as efficiently as a human can, and quickly overheat. This overheating causes them to stop and cool down.
Additionally, the human body is built for running: the Achilles Heal, and other tendons, act as springs that help store energy, the quadriceps are powerful, long distance pistons, and our cooling system is top notch.
So at the 26.2 mile track, my money's on the biped.