In reply to fenski:
What is your current training plan? That is, what is a typical week both in terms of mileage and types of run (i.e. speedwork, hills, trails or road etc.)? Easier to tweak what you have I think.
Also, how did you find the marathon? Did you go all out for speed or were you simply trying to get round? If you were just trying to get round, did you feel quite comfortable with the distance?
In my case, I found a hilly off-road marathon a lot easier than a full speed flat road marathon, because the variation of terrain. Flat road marathons (if you're going for a time) are relentless and more difficult psychologically (for me). Certainly, were it not for some nutritional errors resulting in cramps, I felt like I could have gone further in the off-road hilly marathon, but by the end of a road marathon I am totally spent!
In terms of training, I just moved my long run from on-road to trails in preparation and that was about it. But I was luckly enough to have hills where I live (certainly hillier than the Netherlands!), so I'm not sure what you can do about that. Is there anything (however small) you can run up and down (possibly repeatedly) just to get some hill work in?