The process of building muscle begins the second that you ask your muscles to do something challenging and unfamiliar, whether that’s picking up a dumbbell, performing a push up, or sprinting on a treadmill. If the muscle is challenged, it will change.
What happens if you don't exercise? If you do less exercise or activity you will become deconditioned. Your muscles weaken and lose bulk including the muscles you need for breathing and the large muscles in your legs and arms.
You will become more breathless as you do less activity.
If you continue to be inactive you will feel worse, need more help and eventually even simple daily tasks will be difficult.
Start with a light exercise routine, for 30-40 minutes three days a week, and gradually build up to 60-70 minutes a day for 5 days. You can do this in the morning or after work, depending on your preference