^^Exactly (to Equiniphile's post). I wouldn't want to try it on a horse that had been turned out and not ridden for a few months because they would be out of shape and they might have problems.
However, if the horse is relatively fit, I don't see a problem with it. I'll often ride my horses for several hours at a trot/lope with only a little bit of walking in there and they do just fine.
Just remember to be aware of your horse. If he starts struggling, then slow it down. If he's going great guns with pep in his step, he could probably manage to go a bit faster for a while.