[thaumasiotes]

But what the article says is that longer courses favor the horses. We can't just define "long distances" to mean "exactly 21 miles". "Humans are competitive with horses over long distances" is a terrible way to phrase the idea that the advantage of horses grows with increasing distance.

Consider that cavalry wipes the floor with infantry in terms of how much ground can be covered in a day. Don't think soldiers are selected for speed? Consider also that we have an inscription from an Egyptian pharaoh commemorating the excellence of his soldiers, as measured by the speed of their march.