“When I broke the world record in Berlin, I had trained for the 10,000m on the track too and I did a lot of speedwork,” Gebrselassie said. “I did not win a gold medal in Beijing but the speedwork I put in during training helped me break the marathon world record - and I know I will break it again, soon - 2:02 is very possible.” -Daily Nation
-quote from marathon world record holder Hallie Gebrselassie. Gebrselassie finished 6th in the 10K in Beijing
MY COMMENT: I've experienced this before. Several years ago I competed in the World Masters Track & Field Championships in Brisbane, Australia, entering the 8K cross country, 5,000 meter, and marathon. I really wanted to run a fast 5K, so most of my focus going in was on that, running a couple of very hard track workouts a week. At 48 years of age at the time, I was hoping to break 16:30, however my typical workouts were 10 x 400 in 80, or 1.5 mile time trials in 8:00. I seemed to be able to hold 80 second 400s, but not much better. Sure enough, in the 5000 I went through the 400 in dead list (out of 23 in my heat) in 80, but proceeded to clip off 80s just about every lap thereafter to finish 8th in 16:40, averaging exactly 80 seconds per lap.
I guess I ran close to my potential in that race, and a week later ran a very good (for me at 48) 2:45 marathon, feeling very, very good.
I am not suggesting that you will run a good marathon on track work alone, but once you have a good base of aerobic distance, a dose of short quality workouts can pay dividends in the marathon.