Three things help with older, well worn knees;

1) Favor riding flat ground. Hills are tough on knees and you can get plenty of workout on flat ground.

2) Stick with the mountain bike just long enough to decide if you are going to keep riding. Then find something else that is better suited to your actual riding. "Pushing" gnarly knobbies puts a lot of stress on your knees. Get a bike that rolls better.

3) Spin the cranks in easier gears. Muscling stiffer gears is definitely out. bk