As you grow older, you start losing muscle mass. It's just part of aging. However, if you start strength training, whether it's lifting weights or doing body weight exercises, you can slow down the muscle loss as you grow older (generally, after the age of 30). That's your metabolism you're losing. Muscle uses more calories than fat.
Start off by doing body weight exercises such as push-ups, squats and lunges. I swear, those work. As you grow more advanced, you can start holding hand weights to make them harder (but it's up to you). Or, you can do jumping lunges and jumping squats. Just push yourself (but not too much).
Having muscle is important. Having strength is important. Don't just focus on cardio.