It's not the distance, it is the speed. IMO
the problem is with people, often newbies, who let the horse dictate the
speed. Horses have no idea how far they are going, and so will go
fast as far as they can. They are prey animals, adding
adrenaline causes their genetics to direct them to flee. IME horses can go
a lot further traveling slower than faster. You are much less likely to
get a horse into trouble at any distance if he is encouraged to trot at
a comfortable(for him) rate interspersed with periods of
walking. You can cover a lot of ground without overstressing the horse
this way.
