With the exception of Arabians, and recently some small start up breeders, The thing to keep in mind, is historically and even by biology horses have not been bred for endurance. In the wild horses sprinted to escape predators. Within a mile or so they either got away or got ate. If a predator wasnt chasing them, they simply moseyed along at a walk, form grass to water hole.
With mans breeding again, not for endurance, (talking endurance racing here ),, Horse were bred for sprinting, pulling, easy nature, size, color, dancing, fancy steps, etc. Not much attention to endurance racing outside of Arabians.