Without exercise, your muscles will be weak and you're risking poor health especially as you get older. It's possible to be skinny fat where you store fat in unhealthy places such as around your organs. Lack of exercise increases your risk of stroke, Coronary Heart Disease, high blood pressure, little energy, stiff joints, osteoporosis, poor posture, and mental health issues. Unless your calorie intake is low, you're also likely to be overweight.