Well, a horse can gallop at around 30 miles per hour, whereas an average human being can run at around 17 or 18 miles per hour. So really, there's a vast difference in speed, but I honestly do not know if a human being can actually outrun a horse in a given distance.

About the speed of the horse's gallop, and the speed of an average adult when running are both rough estimations from the results I got on Google. So yeah

Well, I'm not sure about that at all, but I'm sure it depends on the terrain, as well as the fitness of the horse. Otherwise, I don't know