John Tribbia, Mario Mendoza & Max King at the 2010 USA Trail Marathon Championships
Your race day nutrition plan (or lack thereof) can make or break your race. It's that important to dial in your nutrition so you can stay strong through the final miles. One key to getting that right is training with nutrition. Use your long runs to try new things and drop things that don't work. Don't rely on the race's aid stations for your calories. They may or may not have what you need. Your long run eating won't completely eliminate the risk of having nutrition related problems, but it can significantly reduce them.
The proper nutrition is a key element to any training plan. So, make a plan, get some advice, try it out, and go race.