I'm partial to the gaited horses for all trail work, and one you may wish to look at that hasn't been mentioned yet is the Paso Fino. In my experience with them, they are tough, sure-footed horses, with very good endurance and a ride that has to be tried to be believed. Don't get me wrong, I like Tennessee Walkers, but IMHO they aren't in the same league as the Pasos.

Eric