This section is from the "Health and Survival in the 21st Century" book, by Ross Horne.
Physical fitness is a factor in health and longevity but it is not the major factor. Recalling Dr de Lacy Evans' survey of people who lived for a hundred years or more, many of the centenarians were sedentary throughout their lives, and although their exercise habits, drinking habits and smoking habits varied widely, the one thing they displayed in common was that they had always partaken sparingly of food.
The degree of physical exercise needed by any individual to maintain good health depends primarily on the degree of physical fitness required for whatever tasks are required of them, and this standard will be automatically achieved as they become competent at the tasks. Above a reasonable level of fitness there is no added health advantage to be gained (bearing in mind the facts relating to heart disease, blood viscosity, etc) and if carried to extremes physical exercise can be highly stressful and counterproductive to the intended purpose.