This is an excerpt from Runner's Edge, The by Stephen McGregor & Matt Fitzgerald.
While it's obvious that a speed and distance device can be used for monitoring and controlling your pace during races, you need to use your device somewhat differently in races of different distances, and you must avoid succumbing to the temptation to rely on it too heavily.
First, before you race, try to get a good sense of your device's specific degree of accuracy. Most devices are inaccurate by a consistent degree in one direction—either too long or too short. Test your device on measured courses whenever possible to determine its pattern. Races themselves afford some of the best opportunities, but be aware that it's actually normal to run approximately 0.5 percent too far on certified road race courses because these courses are measured by the shortest possible distance a runner could cover in completing it (that is, by running every turn and tangent perfectly), and nobody ever does that.
Pacing During a 5K
If your device model has an option to display the average pace for the current lap or run, set the display in this mode before the race starts. If you're running a 5K, ignore your watch for the first several hundred yards, when it's crowded and your main priority is to find a rhythm. Once you have found your rhythm, take a quick glance at your average pace. It almost certainly will not match your target pace for the first mile, but that doesn't mean you have to actively speed up or slow down. Just absorb the number you see, think about it in relation to how you feel, and let your gut tell you how to adjust.
Sometimes this early quick glance can save the day. When adrenaline gets the better of you and you start way too fast, it gives you the chance to rein in your legs and save your race before it's too late. If you waited until the first mile split to discover your mistake, it would be too late. On the other hand, if you start way too slowly, the quick glance at your average pace may remind you that, in fact, you are not working as hard as you could be, and you have an opportunity to speed up before you've dug too deep a hole to climb out of. But most often that early, quick glance will merely confirm that you're more or less on pace.
Pacing During a 10K
When running 10K races, do the same early glance at your average pace as soon as you've settled into a rhythm and adjust, if necessary. After that point, ignore your device (but pay attention to your mile splits) until the second half of the race, during which you should check the device whenever you find yourself worrying that fatigue is causing you to slip off your goal pace. The benefit of doing this is that it almost always motivates you to run harder, no matter whether the display tells you that you're right on pace, have fallen a second or two per mile behind pace, or are ahead of pace. The only circumstance in which it's likely to be demoralizing is when you're having a bad race and have fallen far behind your target pace. In these circumstances, you're going to end up demoralized anyway.
Pacing During a Half Marathon
Half marathons are long enough that your mile split times become almost meaningless after you've run several miles and brain fatigue has crippled your mathematical faculties. So don't even bother paying attention to your splits after 10K. Instead, glance at your average pace at each mile mark to check whether you're still on track toward your goal. As in 10K races, this type of monitoring is likely to keep a fire under you—there's just something about chasing numbers that makes us work harder!
Pacing During a Marathon
In the marathon, all measures taken to control your pacing with objective data go out the window after the halfway mark. You have to run by feel. But properly controlling your pace with objective data in the first half is critical to setting yourself up for success in the second half. The marathon distance is just too long for your anticipatory regulation mechanism to make reliable decisions about how fast you ought to be running in the early miles. Instead, rely on setting an appropriate time goal and target pace and check your speed and distance device as often as necessary to ensure that you stay on this pace through the first half.
While a speed and distance device certainly can help you pace yourself more effectively in races, it is no substitute for your body's built-in pacing mechanism. While this mechanism is poorly developed in beginning runners, it is highly refined and more reliable than objective pacing controls in experienced runners. If you are ready for a breakthrough race performance, your anticipatory regulation mechanism will tell you so by causing you to feel better than anticipated as you proceed through the miles. It would be a mistake in this situation to trust your pacing plan and your speed and distance device more than your body and resist the urge to run faster. Likewise, on those days when you just don't have it in a race, you need to heed your body's message of unexpected discomfort and run slower than planned instead of stubbornly persisting at your target pace only to suffer a disastrous bonk late in the race.