The reason to do something other than plop yourself down under a tree after a hard workout might not be as obvious as the rationale for warming up. But cool-downs are important.
To see why, think of a graphical representation of a workout. You move from being sedentary to light activity to harder activity to a peak of activity—the meat of the workout, like 800-meter repeats. At that peak, your body’s internal processes are working at a high rate. A good cool-down brings them back down to their normal state gradually, and your body thanks you by being better able to handle the next days of running.
To gradually bring your body down from the metabolic peak it reached during your workout, jog 1 to 3 miles. Follow that with gentle stretching, either dynamic or static, to encourage continued blood flow to the muscles that were recently working hard. This will help to continue to remove waste products and reoxygenate the muscle tissues.