Don’t run the first mile of your race too fast. Even though you always feel the best at the start, pace yourself.
Running too fast at the beginning of a race will cause a dramatic slow down during the latter segments of the race and result in a disappointing finish. The faster you run the first mile of a race, the more your muscles rely on anaerobic metabolism to produce energy. This means that energy is being produced by carbohydrates with lactic acid being the by-product. Anaerobic metabolism is used for short, higher intensity bursts of activity, lasting no longer than several minutes. Following several minutes, the by-product, lactic acid build-up reaches what is known as lactic threshold. At this point, the production of lactate is greater than the amount being removed from your working muscles. This is when muscle pain, burn, & fatigue set in, making it difficult to maintain this same intensity.
For your next race, try to avoid that adrenaline rush that makes you want to run at a much faster pace than normal. Ideally, the second half of the race should be equal to, or slightly faster than that of the first half.
Regardless if you’re running a 5k or a half-marathon, if you start the race too fast, the anaerobic metabolism that occurs during those first several minutes will force you to slow down at some point throughout the race. In the end, you’ll only lose more time than what you “gained” at the start.
Note: I suggest using a GPS as a pacing aid
References: Running Errors and How to Correct Them by Jason Karp