My main issue is with my back
Not too bad but had a couple of minor injuries when I was younger which might be contributing to recent pain- the difference is that in my 20's I'd bounce back in a day or 2 even after an actual injury and now I can experience pain for practically no reason (sitting too long in the wrong chair and/or position or even moving weird while sneezing lol) and it takes closer to a week to feel normal and that's after a lot of self care.
Nothing all that weird about your experience- women in their 40's are generally a lot of fun to date and it sounds like your ladyfriend was pretty youthful in spirit from your description. I wouldn't assume she was taking advantage- sounds pretty mutually beneficial to me. But FTR ladies in their 30's are lotsa fun too and also close to their sexual peak, hence the term "dirty 30's".