Gazelle personally I think 18 weeks is long enough for marathon training, however you should be looking to do a build phase before you start, have a look on the net see what plans you can find. I followed p&d last year for abo, worked really well for me but it felt like a long time. Hill sprints, build up your long runs to 16/18 miles nice easy runs steady runs, there's plenty of things you can get along with. They're called macro cycles but I can't for the life of me remember which one is before marathon training. I think it is a build phase tho.