if you're training for an endurance event like a marathon when you might run 60 miles or more per week you'll almost always see a decrease in your muscle mass. for most of us who do more moderate amounts of physical activity there will be minimal if any loss in muscle mass from aerobic exercise -- so there's nothing to worry about. if you do plan on lots of aerobic exercise and are concerned about losing muscle try starting with 20-30 minutes of moderate aerobic exercise (at 50% to 70% of your maximum heart rate) two to three days per week and see how it goes.
does aerobic exercise interfere with muscle gains from weightlifting?