Cardio alone will not lead to muscle depletion. It would take a calorie deficit to also deplete muscle mass. Take running for example. Your leg muscles and your core work as you run, granted it's not as much as say doing squats or another leg exercise, but they are working.
To lose fat, I would concentrate circuit type exercising, where you do a set of exercises, one after the other, until all sets are completed. This will keep your heart rate up, which will help burn more fat.
As for cardio, keep your distances shorter. Don't think long run equals more fat burning. It's about intensity, do sprint intervals with short rests between each sprint. You will feel the difference in the intensity immediately. That will really get your heart rate going.
Combine those with a slight calorie deficit and you will lose weight, but make sure you are eating the right foods. Also, make sure you are eating enough food, too little and your body will store fat because it thinks it's starving and will save it's nutrients for energy later.