There are 2 things that will make or break a race. Pacing and nutrition.
#1. incorrect pacing, going out too fast or trying to run faster than you are capable of. Hopefully this gets worked out in training as you do your long runs your body gets used to the fastest pace you can hold for the longest time. Typically you don't run faster in a race than you do in training...at least not for long.
#2. Nutrition. Nutrition consists of CARBS, WATER and ELECTROLYES and I will speak of them separately.
CARBS: can be in the form of gels, or gummies or jelly beans or in a powder in your water (note this does not replace hydration). You typically need 250 to 300 calories of carbs PER hour of running which would be 2 to 3 gels per hour. Starting at 45 mins to 1 hour of running and then taking one every 20 to 30 mins AS YOU HAVE DETERMINED works for you during training. Watch for signs and symptoms of low carbs. This will look like your energy dipping, can't maintain pace OR your brain starting to be negative..."I hate this", "can't wait for this to be over", "this sucks".
WATER: we start at 32 ounces (1 liter) PER HOUR of running. If this seems too much you can use the calculation of: your body weight divided by 30 EVERY 15 MINS. Example: you weigh 150lbs, 150/30= 5, that would be 5 ounces of water EVERY 15 mins. The #1 predictor of a bad race outcome is dehydration. Signs & symptoms of dehydration is legs get heavy, you stop sweating, stomach upset, can't maintain your pace, red face.
ELECTROLYTE: this is a buffered solution and can be taken as Nuun or as eLoad Tabs. We usually refer to the sodium content when we talk electrolytes and typically start women at 600mg PER HOUR of running, men at 800mg PER HOUR. Watch for signs and symptoms. If you have to pee frequently (more than every 2.5 hours), if your stomach gets upset when drinking water, if your fingers or toes swell, if your face turns red, if you get dizzy or lightheaded, if you have persistent muscle aches or a post run cough.