I'm sure that you will have many replies and they will all say that it
wouldn't be a problem. The horse would average between 8 and 9 mph and
would need to stop every couple of hours for a drink, ten to fifteen
minutes of the rider's weight being taken off it's back and a pick of
grass. If you want to know about the terrain on Dartmoor - email me
privately.
Hope it helps
Nicky Freud.