Are there any endurance riders in your area? I once went out on a death ride which was typical for an endurance rider that was at my barn. It was on his horse and man all we did was trot. This was over hills, through terrain, and as we approached the top of the first hill I thought, oh good we get to walk. Nope, we trotted straight down that hill and up the next! I had ridden those trails extensively and walked/trotted/cantered them, but never realized the kind of workout they give a horse! I came back with huge saddle sores and muscles aching! I was on a conditioned horse, so you would need to work up to that kind of thing, but hill work is the best at low impact muscling. You can also back up hills to specifically target a weak hind end! I miss having hills to work on and now I am thinking I am going to have to get a trailer out and drive the 15 minutes to appropriate trails. There is really nothing better than making them walk up a really steep hill (cantering and trotting is actually easier for them to go up a hill in). Good luck!