Drica, I'm beginning to think that your problem is that you're pushing too hard and is why you're having the knee pain. I'm not quite sure how to instruct you if your lungs are having a hard time with higher cadences with easy gears but I can assure you that if you're pushing too hard, you are extremely likely to have knee pain. The only think I can maybe recommend, which is what I did for me, is to gradually work at it in what ever gear you need to be in to do a 60 cadence without pushing too hard for a couple of weeks or so. Then try doing a slightly higher cadence and build into it (maybe a different gear). If you continue to have knee pain, then I would think you would most likely damage you knees to where you might not be able to ride. My speed and gear I ride at is all based off of what I can do around and 80 cadence +/- without pushing hard. I started out last year with a Trek 6000 mountain bike. I now have a new Trek FX3 which has much better gears and easier to ride at faster speeds without pushing too much. Your body will tell you, such as your knees, your limitations. My biggest problem is head winds and long uphills but I only go as fast as I can without pushing it too hard...only for my knees sake. I hope I'm not giving you bad advice or suggestions. Everyone's body and muscle tone is different.