I do a lot of cardio and wanted to know how much is good. I'm lean as it is but what is a good amount of cardio to do. I run on the treadmill at about 7 mph with 3.0 incline for about 20 minutes three times a week. It is a really intense workout but now I just want to get more mass. I only weigh 128 pounds and I'm 5'8", what is a workout where I can loss fat but also maintain my lean mass.