Generally, a long run is a base run that lasts long enough to leave a runner moderately to severely fatigued. The function of a long run is to increase raw endurance. The distance or duration required to achieve this effect depends, of course, on your current level of endurance. As a general rule, your longest run should be long enough to give you confidence that raw endurance will not limit you in races. There are many spins you can put on a long run, such as progressing the pace from start to finish or mixing intervals (described on the last page) into the run.
Example: 15 miles at natural pace