Hey, don't be afraid of hills. How do you think these Grand Prix Dressage riders get their horses butts so refined and strong enough to step under themselves? Going down hills teaches your horse to collect himself and get his rear under him ultimately strengthening his hind. I'd love to ride where you are, I hope you're having fun! Just watch some wild horse video's the horse out there have better confirmation then our horses do and they run though everything. As your horse learns to collect just watch what your body is doing to help him and soon enough you'll have learned right along with him. Although I do agree with everyone that has said to start out slowly, its how you build up the right muscles. It sounds like great exercise for both you and your horse! I wish I had your hills, its just flat where I am, haha!