Running is good exercise, but you're going to stress your knees and legs, especially if you're carrying a few too many pounds. I would stick with the cycling and cross trainer. If you can only manage 15 mins at the moment, you're working too hard... Take it easy
The single most important thing that you can do is buy a heart rate monitor, and use it properly. Your body has an optimum heart rate range for fat burning and if you want to maximise your weight loss, you need to stay in this range, you'll actually be surprised how easy it is to maintain this level of exercise, you don't need to work as hard as you think, but you do need to do it regularly and consistently.
Given your current condition and age, I would put your 'fat burning' range between 120-130bpm
Keep your heart rate in between these figures and exercise for 30-45 mins 4 times a week for the first couple of months and you should see steady improvements in you aerobic fitness and (given that you're eating healthily) the weight should drop off.