Eating extra carbohydrates won't help your run faster or longer for races less than 90 minutes. But for longer races (triathlons, marathons, etc.), you'll want to make carbs the focus of your nutrition strategy in the weeks leading up to the race—carbs should be at the center of all meals and snacks. Then as you head into the tapering phase of your training, simply keep your calories and carbs the same as the weeks prior. This will help keep your body properly fueled heading into race day.
The night before the race, eat a balanced meal comprised of whole grain carbs, lean proteins, and healthy fats with generous portions of fruits and vegetables. On race day, have a pre-race snack such as a bagel with peanut butter an hour or two hours prior to lining up. And don't forget to hydrate with water and sports drinks such as Gatorade at each fueling station to maintain your carbohydrate and electrolyte balance during the race.