Public health guidelines focus on increasing low to moderate physical activity levels in a largely sedentary population. While there is some evidence that inactivity is associated with increased risk of morbidity and mortality, there appears to be much stronger and consistent evidence for a graded inverse relationship between physical fitness and morbidity and mortality. However, epidemiological studies investigating physical fitness have often not directly measured aerobic capacity. This calls into question the specific recommendations that assume a direct relationship between aerobic capacity and risk. Performance on some test protocols can be favorably affected by increases in strength and musculoskeletal changes, in addition to aerobic capacity. Other public health recommendations assume that the volume of training, the total amount of work, or caloric expenditure is the key stimulus for health-protective adaptations. However, there is little evidence to support this long-held axiom. A balance of resistance training and aerobic training is recommended for decreasing morbidity and mortality. A threshold theory of adaptation and training is proposed that potentially can efficiently and effectively enhance aerobic capacity and strength in minimal time.