> A multi-day race where riders can disregard stages will turn into a nightmarish competition of inhuman endurance.

Iditarod and Yukon Quest are both like this - I met someone (who had won the Yukon Quest) that started wearing a helmet mushing because he had passed out from exhaustion and hit his head.

edit/P.S. When just googling I saw something talking about if these races were cruel to the dogs. While obviously some people have (or continue to) treated their dogs poorly, I must say his dogs clearly lived to run. They were so cute and energetic. He talked about how a woman a couple decades ago schooled all the men in the sport by truly taking care of her dogs, and it seemed like that had a big effect on how he raised his dogs.