How much should I eat the day before a long run?
On the day before your long runs, practice eating. Be sure to jot down what you eat, and how much. You can fine-tune the plan as you go through the program, based upon what works for you. Then, on the day before your marathon, you will replicate the eating routine that works for you. Here are the general rules:
1. Don't eat a large meal after 5pm the afternoon before.
2. Avoid salty food and alcohol the afternoon and evening before long runs and the race.
3. Eat smaller meals or snacks about every 2-3 hours, starting about 12 noon the day before.
4. Choose foods that digest easily.
5. Drink 6-8 oz of water or sports drink with your snacks.
6. It takes about 36-48 hours for the food you eat to be digested, metabolized and ready to be used in the muscles during exercise. Last minute nutrition “cramming” will not help you during the run.
7. I've never seen a runner die of starvation during a marathon.

What should I drink the day before long runs?
Drink about 6-8 oz of water or sports drink, about every 2-3 hours. Accelerade has been shown by good research to help normalize your fluid levels better than water and another sports drink. I recommend a total of 50-64 oz of fluid throughout the day. Avoid alcohol!

What should I eat and drink before the race?
The eating/drinking strategy that works for you, before long runs, should be your guide. Fine-tune the amount and the timing and use the pattern that works best. Unless you are diabetic or have severe blood sugar problems, you may not need to eat anything at all. If you get nausea from eating before running, reduce the amount or don't eat at all before long runs, and see how this works. Most of the runners I've worked with who do not eat breakfast, start their blood sugar intake at 2-3 miles into the run. I recommend taking 6-8 oz of water, about 2-3 hours before the start. Practice this before long runs so that you will visit the toilet before the start—especially if you drink coffee. Coffee has actually been shown to improve endurance and enhance fat-burning, but you need to find the right timetable for consumption.

What should I eat during long runs and the race itself?
Running a marathon puts enough stress on the system to shut down the digestive tract. Therefore, very little of the food and fluid you consume during a run can be used during that run. It is possible to absorb a small amount of fluid, and blood sugar booster, every 15 minutes or so, and this can help you stay motivated. Your brain needs glycogen as fuel. As you continue, mile after mile on long runs, the available supply of this limited resource is reduced. If you don't consume enough to boost the blood glucose (using gel products, hard candies, gummi bears, energy bars) the mental focus and motivation is reduced. You can maintain mental energy and avoid some of the negative messages at the end of your race by eating small amounts of foods with sugar, regularly. Practice taking several products during long runs to find what works best for you. This allows each runner to discover the best source, the amount, the quantity of water, the frequency, and how to adjust as the distance increases. A rule of thumb, based upon the runners I've worked with, is the following: 30-40 calories every 1-2 miles, starting @ mile 5. Be sure to drink a little water with each blood sugar snack.

From Marathon FAQ, by Jeff Galloway