I've got my second marathon next April. I'm going to do a 16 week training programme for that one, but starting it at 20 weeks prior (experience tells me to expect something to interfer with training....).
From now until then - I want to build back up to half marathon distance. I'm going to run 3 times a week. Doing one tempo run; one threshold run and one slow long run. I want to run 13 - 15 miles as my long run every other week, with the inbetween weeks doing a much shorter long run to rest up. Therefore, when I start my marathon training, I'll be starting from a really strong base line.
There's all sorts of advice out there. Lots of different types of training. It's really confusing, i think, to work out what will work for you and why it works for you. Best advice I got was - don't just clock up the miles; think what you want to get out of each training session and make each session count.