Most people will burn less than 2000 calories in a half-marathon. Your body can hold about 2000 calories of carbs at all times, which can be turned into fuel. If you are eating right, you should not need a "snack" during the race.
Here is the way to make sure you have the proper nutrition... before, during, and after... for a big race. Our good friend, and Bib Magnets user "The Diet Diva", Tara Gidus breaks it down here.