Bones and Strength
Sitting around or lying around all day makes you weak. Unless you continuously use the major muscle groups in your body, they do not strengthen. Bones also lose density with age, and lack of weight-bearing exercise plays a role in osteoporosis, or brittle bones. Your body responds to the demands you put on it, and if you do not exercise, your muscles and bones weaken with time.

Exercise for Endurance
Just walking up a flight of stairs can make you short of breath if you are out of shape. Lack of exercise can lead to a lack of energy and endurance. The listlessness you feel further dissuades you from engaging in physical activity, and the vicious circle continues. Breaking a long period of inactivity is not easy. Starting slow with just a few minutes of walking daily gets you moving safely.

Mental Health
Lack of exercise can lead to a diminished sense of well-being. Your body loses muscle tone and strength and your self-esteem can suffer as a result. Weight gain might lead to social isolation and bad eating habits. Endorphins are natural painkillers that help elevate your mood. Resuming physical activity benefits your mental health as well as your physical health.