Sounds like you are already doing much of what builds muscle and muscle tone over time.
Building of muscle and fitness takes time, weeks to months of it depending upon how often you ride, how long you ride for and the intensity of the work-session.
If you are bringing a horse back from injury, from being off for months then go slowly and build gradually as horses are exceptional athletes who need conditioning effective and carefully done.
You should be able to ask those who are watching you ride and school about issues like this knowing your horses specific needs and limitations.
Here in Florida hill work is not easily done with our more flat topography but there are many endurance horses, skilled working horses in many disciplines...
When I was a kid we use to walk
our horses on the beach in the firm wet sand, walked not run,
as it is strenuous exercise but you said in another post where you are and water is not far from you in that location so might be a possibility to look into.
We never intentionally rode our horses in deep sand/dirt as it is
a fast way to strain/tear/blow or hurt a tendon, ligament or joint from stresses unaccustomed to.
Also remember that not every horse makes bulk muscle or muscle-tone, some are lean, lanky and whipcord lean but muscled and fit...just look at racehorses, that is fit those elite athletes as they are.