trot sets are a good way to work longer and still let your horse rest, as well as dressage and keeping in a frame over cavalettis. To keep my horse in shape and sound we only jump about once a month, but for a horse that needs more work in jumping i would only jump once a week.
If you have any hilly fields to ride in cantering/galloping up and down them helps their muscles a lot
At the moment I am only doing Intro but I still ride him 5 times a week I try to jump him once a week he gets a gallop session once a week and flat the rest of it because that's our weekday. With gallop sessions warm him up like you would xc and if u can find a hill or something near you trot up and down 2 canter normally up and down then Trot again then canter then gallop and do it as much as you think he can take it until he is tired. This way they build not only fitness but there bum muscles as well.
I really hope this helps!!