Damn I wish I had a horse I could stand on and not fall and bust my ass. LOL.
I'd do some stretches, maybe get her to flex by using a treat. I'd also work on her topline, but I'm not sure how I'd do that with an older horse. Does she suffer from bad arthritis or is she okay, well she should be if you go for a 100 mile ride, lol! I'm sure others will give you some better advice.