Sometimes the training is too hard for us but there are other species that run faster or longer without it. The horse goes for a top speed near 90 km/h without external weight —so, individually— and near or more than 70 km/h carrying a person.
There's an event called "man versus horse marathon", where men won the race two times. Although it's another example of how winning could be an effect of don't having a better contender in a near field. The times speak for themselves: the best horse time in 35 km was 1:20 in 1984 with the name "Solitaire", and carrying a person. The times for those two wins were 2:20 and 2:05.
What is better is the interest —with a bit of arrogance— of competition though because horses are much more adapted to run and have longer extremities.
At least the ironman triathlon (3.8 km swimming, 180.2 km bicycle and 42.2 km of marathon) cannot include other species especially since no other knows how to ride 180 km of bicycle within competition times.
Horses are awesome!