The horses also all had humans on their backs. To my knowledge, none of the humans had horses on their backs.

The Western States trail in the California Sierras used to be where a 100-mile horse race took place that horse and rider had to complete in 24 hours. At some point in the 1970s one of the riders decided not to take a horse, and he finished in 23 hours on foot. Now it's an annual footrace that the winner finishes in about 14 hours.

This study analyzes historical results of three different man versus horse races (in Wales, in Virginia, and in California). The data shows that human performance decreases with temperature, but less so than horses, so that 30°C is approximately where the best humans can start outperforming the best horses that year.

I would think that even with 15 minutes of intermittent pauses/checks, that time is still productive for cooling the animal and would add less than 15 minutes to the theoretical total if they were allowed to run the whole time.