That's so awesome that you're going to run a marathon! I'm thinking about running a 5K in November.
Anyway, to answer your question about what to eat. All the reading I've been doing have been stressing good carbs from whole grains (whole wheat, brown rice, etc.) to give you energy for running, lots of fresh fruits and veggies, moderate amounts of good fats (flax seed oil and nuts are mention alot) and lots of lean protein to rebuild muscle afterwards.
Obviously, you would want to avoid candy, cake and other sweets along with processed, fried and refined foods.
I'm no expert obviously but I'm sure someone with lots more experience will be around shortly to share their wisdom.
How far I've come thus far...