Beyond 60 minutes a day, you hit a point of diminishing returns and you may increase the chance of injury, over-training and muscle loss. Trainer John Parillo has always been an advocate of lots of aerobics, even for his bodybuilder clients who are trying to gain muscle mass. Aerobics forces oxygen through your body, increasing the number and size of your blood vessels. Blood vessels are the 'supply routes' that transport oxygen and nutrients to body tissues, including muscles, and carry waste products away for muscular growth, repair and recovery. So, according to Parillo, aerobics can actually enhance recovery from weight training and increase muscular growth by developing the circulatory pathways that provide nourishment to the muscles.
Cardiovascular training is important for good health, for fat burning, and for muscle-building.
Losing muscle has more to do with inadequate diet than with excessive aerobics. You are not eating enough protein. Protein is the only nutrient that can be used to build muscle. To stay on the anabolic side you must eat five to six protein containing meals every day. Each meal should be spaced out approximately three hours apart.