The principal causes of sore knees are seat too low and too high gear. Many people want to be able to put their feet on the ground when sat on the saddle. this result un legs never straightening when pedaling. You never walk with knees bent all the time. When you stop move forward off the saddle. Keep in a lower gear so your feet can keep spinning around without feeling that you are putting any effort into your knees. The amount of calories you burn depends on how hard you work at it. Many people have lost massive amounts of weight through cycling, but not me - I am sure that cycling has improved the standard of my health, and my enjoyment of life.