What are the health risks associated with a sedentary lifestyle, and how can regular physical activity mitigate these risks?

A sedentary lifestyle has been linked to numerous health issues, from obesity to heart disease. What are the specific health risks of prolonged inactivity? How does regular exercise help combat the negative effects of a sedentary lifestyle? What are some practical ways to incorporate more movement into daily life for those who have desk jobs or other sedentary habits? How does even light physical activity, like walking or stretching, contribute to long-term health?