I have been running the marathon 6 times (my last was in 2005). When I am training for the marathon, I  always follow the program below.

I run for 9 min. and walk 30 sec. and continue like this until I am finish (e.g. 30 km.)

Best time: 3:49:23 (H.C. Andersen Marathon, Odense)

Longest time: 4:28:55 (Midtjysk Bjergmarathon, Silkeborg)