I think you have a lot of good suggestions here but I want to underline the idea that slower is better! It takes time to build muscle and unless you are planning to go to the Olympics, take your time. That said, you have to be careful to learn when she's telling you that too much is too much versus "ooooo, I'm feeling lazy and I don't think I want to do anything today". That can be difficult, yet important, to distinguish between. Good luck with your gal!! it sounds like she has a fantastic home.
Hills...how steep? I know not "Man from Snowy River" steep, but what is the optimum grade? I have areas that are very gently sloped to those that are fairly steep (I'm guessing about 70 degree angle). Unfortunately, the steeper ones are longer than the more gradual ones.
The only thing the government needs to solve all of its problems is a Council of Common Sense.
Another option is lunging with the Pessoa lunging system, it really does help to build back muscles and doesn't hold the horse in one locked position like side reins. Good luck and remember to go slow she is young so you shouldn't have to rush her.
Please talk to your vet or get with a sport horse vet who will give you a slow, step wise rehab program for the horse coming back from SI and other issues.
Longeing, circles, turns, anything like that is often completely out at the start of rehab. Trails have uneven ground, might be right out. So might be hills. Everything has to be introduced slowly and in steps when rehabbing a horse.