Hill work is very good at building muscles esp in the rear. Like Corporal had said don't let her run up the hill but walk and encourage her to push from her rear this includes going downhill also. In the beginning just let her walk up the hill at liberty then little by little ask for some collection and extension at intervals while going not only up hills but down hills also. Its going to take twice the work to retrain her because not only do you have to break a habit but then you have to instill a new one. Some "habits" are alot harder to break esp when the age factor falls in. If she was allowed to trot for many years then its pretty well ingrained in her head that this is correct or acceptable. Reprogramming her brain will take some time. (and no trotting is not a fault for most gaited horses not hard wired for other gaits will do this its just what the rider allows the horse to perform if other gaits are evident.) Consistancey (cues/movements done properly) is the number one and I mean number 1 key to correcting her "habits". THis can be a challenge....trust me I know. I was working with this QH mare that had been "cowboyed" (not in a positive term) and had learn some realy bad habits (it wasnt her fault but the owner/rider's fault) and retraining her proved to be a huge challenege for me. She had a bit of a defiant streak in her and some time the battle of the wills was not pretty. I almost gave up on her and went to "lick my wounds". But something about her was screaming to me to give her another chance. I did like this mare, and she was a pretty decent horse to be around on the ground. So I decided to stick with her and go back to basics. I found that the more consistant and eliberate (even to the point of over expressing them) I was with the cues I was giving her for what ever I was asking the less the battle of the wills grew. I was on her each and EVERY time she balked, fussed, and boo hooed about what I wanted her to do. I didnt let up untill I got what I wanted. If she responded in the least little way I rewarded her and let her "rest" and after a while she realized that the jig was up and the battles where to exhausting to keep up and she didnt get to win in the end. So the war was over but the restarting/reclaiming had a long way to go. Reclaiming a ill trained horse (either directly or indirectly) can be the hardest part of the deal. I kept consistancy as my number one officer in this reclaiming and what a nice mare this horse turned out to be. It took me two years to turn her out to be a decent well rounded horse. We went from "no chance at showing what so ever" to winning lower level Hunter classes and in the ribbons WP classes. THe worst thing about this deal is that I had to return her back to her owner.
With my late mare (the rescue I mentioned above) it took about 2 years to get her to where I was happy but that included her weight gain/ suppling, flexing and over all retraining. She wasnt defiant in any manner and learned rather quickly for the most part....she was in her mid to late 20s. I did have the advantage to working iwth her on hills and it helped tremendously. (this mare had hip down syndrome and thus a weakness in her hips and doing hill work and flexing work helped with this matter. Cantering in circles was a no go though due to this malady but on the straight ways I let her go and it was a decent canter.)