Wow, seems surprisingly balanced. I would guess that if it was 50% longer distance than humans would win reliably, and 50% shorter would allow the horses to win?

Think it's more to do with hilly, wooded courses unfavourable to horses and a requirement that the horse's heart rate remain below a certain threshold when inspected by a vet in the middle and end of a course, whereas the leading humans are maxing out their efforts

I don't think the Mongol cavalry would lose races to humans over any distance of steppe