Visualization and mental preparation are important pieces to the marathon training puzzle.
There's a saying among runners that running a marathon is 90-percent from the neck up. I'm not so sure, I would even say 95-percent. Your head has got to be in the game.
You're now into the double digit mileages and sometimes you will do a 12-mile run and you'll have great day and the next week you'll back off to an eight and you'll just barely get through it.
That has nothing to do with whether your body is ready or not, it's all about the mind.