If you google "lose fat not muscle" you will find lots and lots of tips on how to do that. Includes what to eat and what type of exercise to do. One thing it says is to do a type of exercise that builds muscle because your body then knows it has to hold onto the muscle because it's needed!
I have been using an exercise bike for about 3 months now (30 min sessions three times a week) and although I'm not losing weight in lbs, I can feel the weight loss in my clothing. They are going on alot easier (especially the bottom half!). It's not really answering your question directly, but does indicate that there's muscle being built here. And muscle does weigh more than fat (hence why my weight is staying the same yet my clothes are fitting better these days!).
Important to do resistance training too as lean muscle burns more calories than fat even at rest. Combining weights with cardio, generally eating healthily and being careful that you aren't over-eating (or drinking) will get results fast.