Like the other two have mention, going up and down hills at a walk do help. Also, some breed of horses is just lean looking and never gets muscular. Gaited horses are one. Some just have a lean narrow look to them. Also, some horse just is not muscular horse. How often are you working your horse? You need to work a horse more than a couple times a week too. The ideal is to have a fit horse. You start out conditioning the horse slowly and build up to doing harder work. Maybe someone can help you get a fitness program going for your horse. Interval training helps too. The thing is what are you trying to get him in shape for? There is different ways of doing it depending on what type of riding you want to do. Need more information.