Trotting builds muscle while cantering burns more fat. Make sure that you're doing more trot work than canter work when looking for fitness as not only will it help with muscling, it will also help cardio. Some canter is good too as it will build cardio and allow the horse to move out as the trot is the harder gait to do for most horses.
In addition to hill work, you can do trot sets (think like jogging sets for yourself) where you do non stop trot for timed intervals, frequently using circles, serpentines, and changes of direction to keep the horse focused. Between sets, give the horse several minutes of trot work, and be sure that you're not doing more intervals or longer intervals than your horse's condition can handle. Each session you want to increase the workload slightly but not so much that it causes unsoundness issues, and you'll want to vary so that every few days you have a light hack as opposed to a more intense hill or interval conditioning workout.
Remember to take it slow - just as if you were prepping to run a marathon, you wouldn't start by running 20 miles for the first time - and give your horse ample time to build up to whatever your goal workout is.
A lot of horses stiffen up in the winter too. I like to do a lot of stretching and bending exercises to bring back the suppleness in my horse. This will also make other work easier. I like working on circles in the trot. It brings out the muscles and adds to the muscle building aspect by bending the horse and bringing isolation to muscle groups and strengthening.
I lunge my Thoroughbred for an hour and a half on some days. I transition from big to small circles, which teaches him balance and builds muscle. I lunge him with side reins or over jumps or trot poles, too. It has to be a fast, working trot, though, not a lazy little pitter-patter. That's how you build muscle.
Riding in small circles or uphill is a good workout for both of us. XD