It is essential that you try to build up your weekly mileage relatively slowly over a period of time. This allows you to maximize your running sessions by following a marathon training routine comparable to what advanced marathon runners use. I wish you all the best with your marathon training and hope you succeed in crossing the finishing line by following a step-by-step marathon training program that has been proven to work. Ideally you should be following a marathon training schedule that incorporates both long training sessions, shorter training sessions as well as rest days in the program that you choose to follow for your marathon race.