In races of 1 mile and beyond...hold back for the first 10% of the race....then...let your body be your guide.
First...a little background. Most runners have a plan when they run a race. Usually, their plan involves running a certain pace...as most runners know that the fastest way to run any race over a mile is to a run an even pace. Actually, what you want is even effort...but for now let's just assume a flat course with little wind...in which case pace and effort would basically be the same.
Pace is usually measured by minutes per mile. Consequently, when the runner reaches the first mile, they look at their watch and adjust their pace accordingly. If they are ahead of schedule, they might slow down. If they are behind, they might speed up...if they can. Regardless, they use their mile "split" as their guide.
The problem with using the first mile "split" as feedback is that it is often not representative of current pace. For example, a runner may hit the first mile of a race in 6:00. Since runners have an overwhelming tendency to go out at a pace faster than they can hold for the entire race...the first quarter mile very well might have been 1:20, the second quarter mile 1:27, the third quarter mile 1:33, and the fourth quarter mile in 1:40. It puts you at 6:00 for the mile, but your most recent quarter mile was at 6:40 pace. Assuming there is no significant variation in terrain....you are not currently running at 6:00 pace. Far from it. You are slowing down. This is a very common scenario...particularly with less experienced runners.
It's not that your time at the mile split does not provide feedback. It does. It's just that it needs to be taken with a grain of salt and there is a better way to insure you have a good race and get the most out of yourself.
The biggest reason for disappointing races is going out too fast relative to ability. The trouble with using your mile split as a check against this is that by the time you reach the mile...most of the damage is done. It's way too late to fix things. In a 5K (3.1mi), the first 400m is what does people in...not the first mile. It just so happens the mile is your first chance to check your pace.
Starting out excessively fast in a distance race is like borrowing on a credit card. You pay it all back with interest...interest in the form of additional seconds and minutes. The more you exceed your optimal pace, the more interest you pay back....and the higher the interest rate. It's not a linear thing. Exceeding your pace slightly carries a low interest rate...exceeding it by a lot carries a high rate.
Guard against the trap of going out too fast by intentionally slowing down...just slightly...for the first 10% of the race. If the race is 5 miles...stay relaxed, non-competitive, throttled back for approximately half a mile. If it's a marathon, pull back for the first two or three miles...especially the first mile. If it's 5K the first 500 meters (a little over a 1/4 mile) is the crucial period. And so on. The 10% is a general guideline.
Keep in mind...this is nothing drastic. It is just pulling back a little. It's subtle. For example, a runner planning to run 40 minutes for 5 miles might run their first 1/4 mile in 2:05, their next quarter mile in 2:03 and then fall into their planned 8:00 per mile pace. Assuming they are actually capable of running 5 miles in 40 minutes, they will do well. This is just to give you an idea. It's not meant to be exact or nit picky. You have to go by feel.
The way to get good at this is to try it. You have to learn what it feels like to go out relaxed and dial down your pace. Once you do it right, you will always know how to do it. Once you get it right, you'll discover how your body will take over and find the exact correct pace...because you let it.
This technique is really a trick. It's nothing more than a way to trick your ego...or maybe it's a way to discipline your ego. Either way...it works. It works because early in a race...the first 10% maybe...the ego is in charge. The ego wants to be a big shot...special...better than it really is. But after the 10% period...the ego recedes and the laws of nature and your own body wisdom assert themselves. If you keep the ego under wraps...during that "danger period" where it is trying to run the show, all will go well later. If not...get ready for double digit interest rates.
Some people ...either due to training methods or genetics...can recover from an excessively fast early pace better than others. But most of us pay a dear price. The good news is that there is very little penalty for going out slightly slower than your ideal pace. In fact, it usually works out better. Also, the longer the race, the more you can lag the pace early on. In a marathon for example, you can "give away" a minute or two in the early going with no negative affect on your result. In a two mile race...you can't dawdle around...you've got to go hard. But you can still back off a second or two in the first 300 meters.
So there you go. When your next 5K rolls around, tell yourself to stay a little pulled back...not lollygagging...but well within yourself for the first couple minutes...rather than charging out of the gate. Remember...when it comes to running...the best tactic is to get to the finish line in the least possible time.
PS...Here are some splits from my 1991 NYC marathon. I was hoping to run 6:00 per mile (2:37:10)
mile 1 - 7:05 (uphill)
mile 2 - 6:25 (downhill)
mile 3 - 6:08
Time at half marathon - 1:18:48
Finish time - 2:36:48 (finishing pace = 5:59 per mile)
PPS They weren't all like this one unfortunately. I went out about a minute faster for the first 2 miles in 1990 NYC Marathon and ran 2:44:34. Learned some lessons the hard way.