Horses can run at 30mph - so a human racing a horse might seem like a no brainer. The horse is going to win.
Except, that's not how it works out once you extend the distance. In the 25 mile Man Against Horse Race in Prescott, Arizona in 2016 the times for the winning runner and winning rider were...identical.
The horse did win the 50 mile race, by quite a bit (90 minutes), but there's more evidence. The average speed of the winner of the 2016 FEI World Endurance Riding Championships over 100 miles was only slightly faster than the average speed of the winner of the men's marathon in Rio.
Humans are pursuit predators. We are amazing endurance runners. A horse can keep up that 30mph (or even faster in the case of specialist racehorses) for about four miles and then that horse is, well, done. Zzzzz. Rest now. Endurance racers alternate walk and trot, and only canter or gallop for short periods. In many races, they're mostly walking. The human, on the other hand, can keep up a nice steady pace for the entire distance.
Overall, it tends to even out - and humans and horses cover about the same amount of ground over long distances.