In my experience you only start to lose muscle after more than 30 minutes of cardio and if you do weight training, which is a reason for your body to hold onto muscle and get your daily protein requirements you're less likely to lose muscle. I would say if you're going to use the treadmill for your cardio hit it as hard as you can in those 20 minutes, but try to change it up as well because running isn't the best way to lose fat, especially if you're doing it after every session. Do some research and find some HIIT that suits you.