The most important thing in running, is pacing. Most runners make the mistake of going out really fast for the first half of the race, and then the second half suffers. If you have a goal of running say a 10 minute mile, it's best to run the first half at your ideal pace, so run the first half mile in 5 minutes and then repeat it. It's much easier to run a pace, it takes awhile to learn, but that way the second half of your race doesn't suffer, and you may even run negative splits (which is a good thing - when you run the second half faster than the first). So say you're doing the 10K, run the first 5K at your ideal pace of 6 mph (or 10 minute miles) and then do the last 5K at the same pace or faster.