Trail riding is great for the mind, balabce, and gettin them to pay attention to where they put their feet
I would start with a few weeks of just walking (outside of the ring/on hills if possible), and slowly introduce trot intervals. Once you start seeing some muscle develop you can focus on working in a frame/collection/canter work.
Kinda the same thing as Caroline Shook said. Make sure you also feed plenty of bulk food so that way the horse has fat to turn into muscle, as well it also helps the horse stay refreshed and energised depending what you feed. The first few rides I'd say just do w/t. After a few rides then slowly bring the canter into your work for short periods. Adding some trot poles in after a few weeks will help him gain more muscle and add interest into the ride so he doesn't get too bored.
Chiropractic can also help improve balance and stability.
start your horse slow just by lunging. gradually increase the amount of time you lunge each day and give a rest day once a week. have one day where you do a little less then normal each week as well so that you are not pushing your horse hard every day. start riding after about two weeks. w/t/c and gradually increase time over the weeks. at least one rest day a week. if your horse seems like they are struggling with the work, finish on a good note even if you haven't done very much. the goal is to get your horse comfortable working.
For a horse to carry a rider AND excercise he will need stamina and strength. So you need to redevelop both areas.
I usually do 1-4 weeks on the ground only and then I start riding slowly as if it were a green horse.
Progress is determined by the horse. He sets the timeline, if he's tired, you take a break. He's like any other athlete.
Endurance is built by a long time at a gait while strength through constant variations in gait, such as canter-walk and jumps.
Trotting poles and cavallettis mixed in between are also very effective.