can you please tell me how you conditioned your horse for driving?The CDEs I have done are about 5-6k, done at a 9kph trot. So roughly about half hour trotting. Usually a few hills, mostly on grass, sometimes road. Neither of my boys had any problems, never held at vet check, never too tired. I always make sure they are properly conditioned.
When I trail drive with friends, we are usually out about 1 1/2 hours, stop for a lunch break, then another hour or so. We do some trotting, and lots of walking because we are doing lots of talking!
And then there's the National Drive, where we can be out several hours each time, twice a day, for several days in a row. I always am aware of how my horse is behaving. If he seems too tired, I turn back or skip a time. It's our vacation, so we want to head home tired!