Another over 75 backpacker here.

Start slowly and work up gradually. You need to develop muscles to support your joints, and exercise to increase the range of motion of those joints. The physical therapy is an excellent idea. Don't expect to get up Mt. Everest this year--maybe the year after next? Don't expect high mileage anyway--my limit is 5-6 miles per day, but that doesn't keep me from getting out and enjoying myself!

If your physician thinks that arthritis means you should limit your activity, it's time to find another physician!

Read the articles on the home page of this site, left-hand column. on reducing backpack weight, paying special attention to the 27-pound, seven-day gear list. If it weren't for those, I would no longer be backpacking!

If, like me, you are pressure sensitive, get an insulated inflatable sleeping pad. There are lots of different styles around; try them in the store. Whatever you buy and take home, spend several nights on the floor with it until you're sure it's comfortable for you--take it back if it isn't. A hammock is also a possibility if you don't camp above timberline.