My interpretation is that the second pair of legs (arms) only gets in the way, if you want high speed on a plain. That is true for both theropods and primates, just look at a chimpanzee running...
Horses and cheetahs both seem to do fine, and much better than humans.
Also, the human body-plan seems set up such that a running human has dynamic instability: we run "better" by pumping our arms around, keeping balance with the shifting mass rather than purely relying on our legs to do that job.
Horses (and various ruminants) are a bad example because they need a relatively big mass of fiber in their digestive tracts. Can't do that on two legs. Still, their fore legs are considerably lighter and contribute relatively little to forward motion.
For that matter, a human can run down a horse. Some Africans still do that with zebras. Cats can't do that.
And no, "pumping" arms around is not really a necessity.
Again, Humans have done this as a hunting strategy for at least as long as there is H. sapiens sapiens. Pick an individual animal, run after it until it collapses.
I guess the horses they hunted weren't as well trained, bred and fed as those in sports competitions. Wild horses in general evolved to run occasionally over relatively short distances.
Yeah, I wasn't posting it as contrary evidence. Note the part where humans do win.
Several humans working together in familiar territory would likely be able to start an animal sprinting several times in fairly rapid succession and avoid the marathon.
They run better over short distances. Bipedal great apes do better over long distances. This is proven both by experienced hunters and myself as I hunt down my cat for kisses. After just a few minutes, she hunker down and accepts her fate since I can literally jog around my house all day.
