The long run is actually a smaller piece of the pie in the marathon. For me most successful marathons are building the pace & cruising speed. Lots of 12 -14 mile runs and the occasional long run. I like to go into a marathon with pretty good aerobic power before the specific training. I have always felt weekly miles trump the illussion that the long run is the key weekly workout. It is important, but every 2-3 weeks is enough for me.
I was able to take my ultra training and then roll of that into faster pace/strength type program and be fairly successful.