I've always had an easy time maintaining muscle mass. For me all it takes is reasonable blood sugar control and endurance exercise that's fun - like paddling, rock climbing, hiking, cycling .... One thing I've discovered however, now that I'm in my mid 50s, I tend to lose muscle mass much faster if I don't do anything for a couple of weeks but I've been told that's normal. Find a fun recreational activity that doesn't feel like exercise that you can do for a few hours a week and I suspect you will start noticing a difference.