Good for you funky24. I've just started exercising again myself and like you struggle with certain exercises due to my weight and my poor old joints. Try not to worry about how many calories you are burning, just moving around is exercise and will require energy. Getting your heart rate up with low impact aerobic activity, like your exercise bike, for around 20 mins will not only burn calories but your body will continue burn calories afterwards.
Be gentle with yourself; 20mins gentle cycling will do you more good than 10 mins of cycling like Victoria Pendleton.
I found a few videos on Youtube helpful to follow. There're helpful for showing various exercise routines, timed classes and for the motivating music that usually accompanies them- it all helps take your mind of your aches n pains. I've just had a look on Youtube and noticed there's a few cycling workout videos.