why is there so much attention paid to increasing a sedentary person's physical activity by a small amount rather than suggesting strenuous exercise?...

From a public health standpoint, why is there so much attention paid to increasing a sedentary person's physical activity by a small amount rather than suggesting strenuous exercise? 

2. How do elderly people respond to exercise training compared to younger adults?

3. What is the effect of exercise on appetite and body composition?

4. What is the role of environmental factors, such as altitude and heat, in very long distance performances of one to four hours duration?