In a recent scientific study just published in the Journal of Applied Physiology, Bangsbo and co-workers demonstrate that by reducing the volume of training by 25% and introducing the so-called speed endurance training (6-12 30-s sprint runs 3-4 times a week), endurance trained runners can improve not only short-term but also long-term performance. -Runners: Train Less and Be Faster (Nov 2009)
MY COMMENT: I'm not doing this! "the runners improved their 10-km time by 1 min from 37.3 to 36.3 min after just 6-9 weeks of changed training. Six of the participating 12 runners obtained a new personal record on the 10-km"
Logic tells me this will help all the way up to the marathon distance!