Yes, you certainly are, but it's going to be quite a bit harder at your age. You have hormones working against you, probably poorer sleep, one or two disorders or co-morbidities...? But I admire your spirit and drive. You CAN gain back muscle mass. In turn, it will keep you warmer, and it will help you to slow weight gain that is age related. Those extra muscles will need fuel and oxygen, that latter of which comes from good lungs and an efficient heart. Don't avoid some aerobic activity, and of course that's at a reasoned pace, not all-out or too hard for too long. Slow stair climbing does wonders if you have some considerable stairs nearby. The pace will almost seem too slow to be of any good, but by the time you're into your fifth or sixth climb or landing, you'll begin to appreciate the actual work being done.