First of all, I am living proof that you do not need to practice drinking or eating GU’s during the actual event. It is no big deal. Here is a link to a great article by Owen Anderson and if this doesn’t make you change your mind about whether or not to consume some sort of supplement during the race I don’t know what else would.
I had heard from another friend (a 2:35 marathoner) about the article and how he had used it for every marathon with great success.
Basically it involves drinking Gatorade (or another similar electrolyte replacement drink) starting right before the marathon and drinking continously throughout the entire race. My friend told me that I would finish the marathon and have to pee immediately and then every 15 minutes afterward for the next few hours. I couldn’t believe it to be true…until I tried it.
Drink Gatorade at every station you can until you can stomach it no more. This usually occurs around 21-22 miles for me. After that you will need no more.
Another method is to use water and GU only, no Gatorade as Owen describes. Getting the correct GU and water amounts is the tough part and why an engineered drink like Gatorade is better.
Personally I could not fathom running a marathon without drinking or eating something as it is scientifically proven that you will run out of glycogen as Ryan stated around 20-21 miles assuming you run the marathon fast enough.