[prestonbriggs]

If it's hot (say, above 70 degree F), a well-trained man can out run a horse over a long distance. If it's cold, the horse will win. The horse's cooling is largely dependent its breathing which is constrained by its pace.

[nthcolumn]

FEI Endurance horse run to 100 kms in over 15 hours. Ultra-marathons of 100 miles are won in just over 14hrs. In UAE, in Slovakia, in Wales. In all conditions, man beats horse over distance.