Usually considered, by most fitness professionals, to be the most important component of physical fitness. Basically it involves the cardiovascular and respiratory systems and their ability to adapt to and recover from stress and physical activity. Cardiorespiratory endurance is the ability of the body's circulatory and respiratory 
systems to supply fuel during sustained physical activity (USDHHS, 1996 as adapted from Corbin & Lindsey, 1994). To improve your cardiorespiratory endurance, try activities that keep your heart rate elevated at a safe level for a sustained length of time. The activity you choose does not have to be strenuous to improve your cardiorespiratory endurance. Start slowly with an activity you enjoy, and gradually work up to a more intense pace. 

The best way to maintain or increase your cardiovascular fitness is to find a form of aerobic exercise that you enjoy. This can be swimming, running, walking, biking, or another activity that raises 
your heart rate to a sufficient level. 
 

Performing CRE type exercises has also been linked with reducing the risk of getting life threatening diseases, such as heart disease and type two diabetes. Low levels of cardiorespiratory endurance (CRE) have been linked to a marked increase in risk of premature death due to several causes, but primarily heart disease.