In reply to KieranT: My approach to this was to google around for marathon training plans - you'll find quite a few on race websites and they're often set up for different target times which is good. I think Hal Higdon's website was one that I used, too. I then looked at what elements the various plans had in common (e.g. long easy-paced run once a week, some strength and speed work) and made my own plan that suited me. For example I wanted a plan based on run time, not distance because I'm often in different places.
I'd also say that for most people a full marathon is quite a step up from a half so you're doing the right thing to start planning at this early stage. It might also pay, (if you haven't already) to read up on nutrition and have a good stretching routine - you're looking at getting your body through a large volume of work and avoiding injuries and illness is important.