Funny Cide (the 2003 ****ucky Derby winner) ran a 2:01.19 for 1.25 miles, which puts him at about 37.5mph
So for the distance no man can run that fast, and I dont think that sprinters can hold up that speed for that long of a distance
I personally think that if a horse is in shape, then it can beat a mtb on the same course, saying that it is rideable by both
just my thoughts