I always felt it was another race. Take inventory of what you have and train accordingly. Mix all the pieces in you can optimize and let fly. Grind down what you think you can run the entire marathon at. Know what you can handle. Too many people try (who want to PR or hit a certain time) and do a big build up during the period you need to be cracking down. Work hard, go easy, work hard, go easy, work harder, etc. Trying to maximize miles and long runs the couple months leading up to the race leads to tiredness and the “am I ready yet,” mentality. Spend the time getting ready, know what you can pull, and take off accordingly.
For instance, if I had averaged 40 miles a week for months and decided to do a marathon in three months, I would try and maximize at 50-55 miles a week all geared towards the damn marathon. It would be futile for me to jump to 70-80 and try and get the best time. That does not work for me. You end up tired and wonder why your not getting any faster.