I run about 3 days a week, about 5 miles. I do the run at a very high paced speed and include sprints and stairs. I am concerned that i am going to lose muscle mass, is this going to happen? I Lift 4 days a week, full body. I lift for about an hour, i lift very hard and heavy. Is this a good combo, my cardio and my heavy lifting? Or am i losing muscles doing this?, hope not!