For marathon running there are lots of schedules around and everyone you talk to has an opinion. However here are some suggestions which are generally accepted;
(1) Have a schedule for 3 months before the race.
(2) Run 2 sessions in the week for improvement, i.e. interval training (for marathons intervals should be 1km not shorter), hill repetitions, tempo sessions (arguably more important for marathon training than interval work), plus one long run building up to 20 miles. Some people also do a short recovery run as the session a day after the long run.
(3) Plan to build up to run 20 miles once or twice (usually once for a first timer) about 4 or 5 weeks before the day then tapper distance down.