I'm a huge fan of proper nutrition before and during a race. With the right nutrients you can drive your body farther and faster than if you just eat garbage. To put it succinctly, your body will take more abuse. And let's be honest with ourselves, the Classic is all about abusing your body. Not in a masochistic manner - more along the lines of drawing on all possible physical reserves to reach a certain goal. Classic racers are known for their ability to force their bodies into extreme endurance. Many a Classic race has been finished or won based purely on strength of will alone while ignoring pain and fatigue. Your body will hold up to this abuse much better if you give it the nutrients it needs.
You will travel farther, longer and faster. You'll live longer and your mind and body will be more balanced if you eat right. So, having extolled the virtues of proper nutrition enough I'll get to the point. I came across a great book about sports nutrition the other day. It has some really good information about endurance nutrition. Some of it is a bit basic but even that is all a good review. Here's a link. Sports Nutrition for Endurance Athletes
Speaking of sports nutrition, here's another book that is based on endurance alpine climbing but has an excellent section on nutrition. The cross-over between the Classic and endurance climbing is obvious and the nutrition fundamentals are very similar. I'm not sure it's worth it to buy the book just for the nutrition section because it's fairly short but if you like climbing books anyway you should take a look. I refer to this book more for the nutrition section than anything else. Extreme Alpinism