Several years ago my TB mare needed to build hindquarter muscle after an injury. The vet recommended I walk her over ground poles, then build to low cavaletti. Placed the poles in different patterns on the ground and then we walked different patterns. We started with 10 minutes once a day and built up to an hour plus twice daily. This was all done from the ground and at a walk to start, slowly working into some slow trotting. Not only did she end up with much improved and great looking hindquarters, I did, too! I suppose you could lunge the horse, my mare needed to go in straight lines, so I led her.
The other thing that helped was the best nutrition I could give her – high quality hay with oats and beet pulp.