Alberto Salazar, the former American marathon record holder and a 10K stand out before that, once claimed that the basic difference between training for the 10K and training for the marathon was the addition of the long run.
Importance of the long run.
Long runs help a runner to build endurance. For best performace the long run should be run once per week, preferably on weekends when you don’t suffer time constraint. The more you run 16 mile – 20 mile the more your body get used to long distance running.