The greatest benefit of a regular exercise program is an improvement in overall fitness. Appropriate exercise improves muscular strength and endurance, body composition, flexibility, and cardiorespiratory endurance. The level of maximal oxygen intake or cardiorespiratory endurance is not by itself as the fact that one's sustained energy-spending ability is directly related to maximal levels of performance. For example, consider the simple task of walking at a rate of three miles per hour. This task involves an energy expenditure of approximately three times the resting metabolic rate. For extremely unfit individuals a three-mile-per-hour walk requires half of their maximal capacity. For a   middle-aged person who exercises regularly the same walk will represent only 25 to 30 percent of maximal capacity. This example illustrates how any submaximal task is relatively much easier for the conditioned individual. Moreover, a person cannot work throughout the day at much more than about 20 percent of maximal capacity without becoming chronically fatigued. Low capability for sustained energy expenditure can support only a very sedentary existence.

The point of the preceding discussion is that the average energy-expenditure requirement of anyone's life can be calculated, and a person's maximal cardiorespiratory endurance determines how active a life-style can be sustained. Individuals who attempt to lead more active lives than their fitness level will support become chronically fatigued. Persons with adequate or optimal fitness levels, on the other hand, are able to meet the physical demands of an active life relatively easily. One of the most frequent observations made by individuals who have begun an exercise program is that they feel better, and research studies document an improvement in feelings of general well-being in more active people.