Are we talking nutrition during training? or during the race itself?
To be very blunt, whilst training, ensure you get plenty of vitamins and minerals (as many fruit/veg as you can manage) and complex carbs to keep you well fuelled for your many training miles.
During the race, stay hydrated (but don't get carried away), and I'm sure others with experience will be able to advise far better on the best way to go about taking on carbs during a race - be it in gel, drink, or solid format!
I recently ran the Nottingham marathon taking on nowt but ripe (and I mean ripe - black spots are essential) bananas (if you can arrange to have friends/relatives cunningly positioned to hand them to you!!). I felt amazing - like I could have kept going for longer...a great, digestible, glucose-rich power source - Nature's lucozade! And no artificial nasties or wierd food colourings to boot! However, I don't know how successful that would have been if I'd been pegging it. I managed just over 4 hours, so comfortable pace all the way - but perhaps if you're aiming for an ambitious time, you'd need a more concentrated supply of energy...Never tried these gels or owt as they simply don't appeal! It's really down to personal preference. You certainly don't NEED them though.
While training - a word of advice. People will tell you "ah you can eat LOADS - you're training for a marathon!" And you will probably say that to yourself too, especially when you want that extra biscuit/bag of crisps!...
Sadly, it doesn't work like that!
Keep your calorie intake roughly the same, with a good hearty balance of carbs and protein - the last thing you want to do is inadvertantly put on weight because you 'deserve' it (think of strapping a bag of sugar to your back with every 1.5lb you gain - and then having to run 26 miles!). Just eat sensibly - not overeating will get your body used to endurance runs on a 'normal' amount of fuel, and that'll mean that you're less likely to hit 'the wall' when you give it everything on the day.
Carb loading the day before though, and the morning of the race, can't be disputed. Pasta for lunch and pizza for dinner on the Saturday, with muesli and toast with peanut butter for breakfast the next day, was an absolutely magic combo for me. You just have to find out what works for you!
Good luck! You will have the most amazing day! Makes all those boring, lonely, (sometimes regretfully hungover) Sunday morning runs worth it!!