Personally, I think it is good to run for a bit and then do some other aerobic exercise after wards. So I think you are doing good by doing the stair stepper after the treadmill. It's always good to work your muscles after they have already been worked. I know stronger muscles burn more fat than weaker muscles, so it's always good to build muscle! As, a woman, just don't try and get too big!!

It is also a great idea to mix up your workout with the treadmill and the stair stepper because your body gets used to a certain motion or movement... this is called the "body adaptability." Having a variety of cardio workout will keep your body from getting used to the movements and enable you to be the most efficient.

As for your question about the bigger muscle burning more fat... it is true... muscle burns fat... even when you’re sleeping... how cool is that! Building your big muscle groups is easier than building small muscle groups... So building up the Quads and Chest will give you more results early on... but ultimately you want to develop the entire body... not just one part of the body. Another good multipurpose exercise along with the stair stepper is the row machine.