After warming up, trot downhill, and canter uphill. It gets your horse in shape!
Walking up hills is harder then trying they have to get under themselfs more
It really depends on what equipment you have available. If you have a euroscisr you can trot your horse starting with 20 min and adding 5 min every week. I would put him in every other day. If not you can do the same thing mounted. The ideal thing for muscle building is hills. Trot up them and walk down. If you don't have access to hiss you can do transitions.