I rode with a friend recently who was riding a mustang/quarterhorse cross.
I said the same thing regarding Mustangs being good horses for endurance
to which she replied, in so many words, that Mustangs are survivors. Their
main purpose is to survive in the wild. That being the case, they would
tend to conserve energy and be more efficient. They are probably a little
smarter than the average breed because of their naturally wild habitat.
They have not been domesticated as long as many of the other breeds. They
would tend to be more like stallions, I would assume. Basically lazy,
conserving energy until a danger arises (or in the case of a stallion, a
fresh mare.) I think what really sells these horses are their heartiness
and good, solid feet. I am intrigued by the breed.