I will probably end up in the woodshed but here's my take on your situation:
I'm the same age as you but the diffrence is I have been riding since I was two - lol Meaning, I am trying to speak from a lifetime of experience and hopefully some common sense.
If it is not your intent to go into the show ring:
1. Agree with Deserthorsewoman to stick with a western-been-there-done-that, seasoned horse.
It always confounds me when I read about older women being put up on Warmbloods in teeny little English saddles. Me thinks you'd have a better chance at learning/finding your balance sitting on a nice-whithered horse bareback. I don't mean whithers that will make you feel like you just had surgery or the horse is so broad you tipple off like a Weebles People.
2. Have you considered a gaited horse? Many of us life-long trail riders went to gaited horses years ago because they are so much more comfortable to the back/neck/shoulders.
If gaited horses are not common in your area, however, that means getting "informal-formal" lessons on one would be slim and none.
If there aren't any gaited trainers around, then I agree to stick with smaller horses and learn the more relaxed way of riding, which is western.
If going on a nice trail ride is your main goal, the only "proper riding positions" you need to worry about, are those that make you keep your balance in the saddle.
One of the first things I taught small children, was first to jog around the round-pen bareback, then with their feet out of those western stirrups, to ensure they had found their seat, therefore their balance
^^^My Arab, now 27, was a great lesson horse with those "nice withers" and just narrow enough thru the back to make holding ones balance pretty easy
3. You sound like you might wear a "no-fear" t-shirt to some degree, which is of great benefit to you but, I also agree with the comments to sort of whoa back and let your previous injuries heal up.
It's a horrible thing to reach this age and not have the abilities we had, even 15 years ago but, pushing ourselves beyond our limits went out the window with our 50th birthdays
Again, you don't sound to be a timid/fearful person, so I say "keep on -keepin' on" with the lessons but on a smaller horse and in a western saddle