Adding muscle isn't an overnight proposition. I start mine, assuming they are mature and in reasonably good health/condition, with about 15 minutes a day either round pen, longeing or working off the golf cart, and build up from there. As work and fitness increase, their feed should increase as well. Use a good quality feed & hay/alfalfa. There are products on the market too that help increase muscle. It is a little different issue since they can't be ridden - I love my golf cart for that reason, it's as close as I can get to taking them out on the trail, gets them out of the round pen and away from constant circling. We lap the pastures, go up and down hills, and everywhere the cart is able.
Jan