If you find it hard to walk, jog or cycle for more than 15 minutes at a time why not try swimming. Swimming is a perfect way to take the pressure off your joints whilst still providing strength training. Work your goals on laps not time. For Example, by the end of the session I must do 2 laps of freestyle, 2 laps of back stroke and 1 lap of breast stroke. Start your sessions small so you can work towards a goal.