Maybe you can find another kind of exercise that won't be so hard on your joints. When I suffered from tendonitis, the doctor recommended swimming. Now I love to swim and it's a great stress reliever. Try yoga or low-impact exercise. I've heard that elliptical machines are supposed to be great for arthritis.

Walking is great, but try to do a bit less (12 miles is a lot for daily) and find something to supplement it. Maybe you can try other non-exercise activities like scrap-booking or crafts that will relieve stress. I do jigsaw puzzles.