Okay, I finally can't stand it anymore (pause while I put on my well-worn nerd glasses):
Horses weren't created/didn't evolve to run (evolve, in my opinion, for what it's worth). They were created/evolved to spend the majority of their time walking slowly over very long distances (the hoof is a marvelous adaptation for that-- it's essentially a big fingernail, so they are literally walking on tip-toe). They are capable of running very fast, yes, but for relatively short distances and in order to evade a predator (so are most hooved prey animals). They are designed to spend most of their lives walking slowly with their heads down, cropping a lot of a low-calorie food (browse). Yes, they can run for stretches in the wild and they do, but that's not an optimum state for the horse-- it's usually a response to a dangerous situation and ends when the danger does.
Yes, I know we've bred the TB to have the desire to run (and boy, do they!) but it's erroneous to start with the impression that the wild horse is designed to spend most of it's life in flight, regardless of the surface. It's not. I think if they were, A) their digestive system would not be as simple as it is and B)our domesticated horses wouldn't need so much cooling out time after a long exertion.
Okay, taking off the nerd glasses now and bracing for the hate mail.
