My old BOs were endurance racers. The had Arabians that were very fit. Their idea of exercise for them was a 10 mile ride of just trotting. Lots and lots of trotting gets them in shape pretty fast, apparently. I went with them one day and made it about 7 miles before my ankle kept rolling (I wasn't using my saddle and the stirrups were up a notch or two too high) until I had to stop.
It's more popular around here than I would have assumed. More in Minnesota, but I still hadn't figured it would be a big thing up here.