I consider myself a good source of information on this topic.
My improvement in the marathon is as follows:
1989: Cleveland 2:44:12
1989: Cal International 2:40:57
1990: Cleveland 2:27:12
1990: SanFrancisco: 2:28:40
1990: Santa Monica: 2:31 (1st place)
1990: Columbus: 2:23:46 (I was ready to go sub 2;20 but I went out way too fast, 15:40 for first 5km.)
Then I had my 10 bad years, and I did not run another marathon until 2000.
2000: Rotterdam 2:36
2001: Amsterdam: 2:38 (training run)
2002: Pattaya (4th place behind a 2:12 Kenyan)
2002: Berlin 2:18
There are 3 things that can improve your performance in Marathon Running. If you have taken any rudimentary course in Exercise Science, you know what they are:
(1) Intensity.
(2) Duration.
(3) Frequency.
(1) INTENSITY. Your intensity level on important workouts should be quite high, but no so high like you are trying to run a sub-4 minute mile! In 1990, I was very ready to run under 2;20, even though I had completed ZERO interval workouts during the previous 6 months. All my important workouts were 10-20 miles around Marathon pace....in other words, focus on a weekly workout of say, 20km at about 5:30 per mile, or 3;25 per km.
(2) DURATION: The marathon is 42km long. 200's and even 400's arent going to help a lot. They are too short. During my build up to Berlin, I did only 1 set of 400's in July. Every other interval workout was 1000meters or longer. During my buildup to my next marathon (Hamburg) I am going to try longer stuff such as 3000s, on the road.
(3) FREQUENCY: Do a tempo run at least every 2 weeks, and a long run, 32km or more, at least every 10days-2weeks. Interval stuff no more than 2 times a week. I strongly believe that the continuous workouts, long runs and tempos are much more important than intervals.
Let's say you are a 2:36 Marathoner, wishing to get down to about 2;22 to 2;25. HERE IS WHAT YOU SHOULD DO:
(a) MOVE TO ALTITUDE. Of course I think that you should move to Lijiang, China, but if you cannot do this, then go terrorize Wejo in Flagstaff. I've never been to Flagstaff, but I think that Laramie (7200feet) is a very cool town.
(b) Take it up to 100miles a week and higher. You are going to get sore if you are not used to training this heavily, but you will get the rewards in about a year or so.
(c) TRAIN HARD! Do a set of 1000s or 2000s every week, and a 20-28mile run every other week, and a hard 15 miler on the weeks that you do not run the long runs.
Jason