As training progresses, it's important to add different elements to your runs. Things like hills and intervals. It's easy to go out and knock off a few kilometres, but if you always go out and run about the same pace, your body will get used to this pace and that will be all that your body will want to do on race day. Now, if that's your whole goal, that's fine, but if you want to have a little extra "oomph" in your engine, then your body needs to know what that "oomph" should feel like.
What exactly is "oomph"? Why, it's that bit of speed that you find to sprint towards the finish. Or the bit of speed you find to pass someone. Or it's the bit of speed you find when you see a bear and know that you need to get out of where you are really really quickly. Or it's the bit of speed you find when you've just soaked your husband with the hose and know he's about to come after you. Basically, it's the speed you don't usually find yourself running on a day to day basis.
So, how does one practice this extra bit of speed? Why, through intervals of course. If it's your first time doing intervals, it's important that you have been running for a few weeks. You want to have a good base so that you know your muscles and joints can handle the speed. Speed is hard work and puts lots of stress on the body, so you want to know your body is in a condition to take it. Best thing to do is warm up a bit, about five or ten minutes of walking, slower running etc. Then, you begin some sort of regimen of repeats. What this means is that you will be running fast for a short amount of time or short amount of distance. Some programs go in seconds and minutes while others are in metres and yards. It's up to you really. At first you might do 5 sets of running one minute, recovering two minutes (so you would run harder than you usually do for one minute and then spend the next two minutes walking or running at a slower pace to recover). Then, you do this again until you've done it five times, then make sure you warm down at the end. If you prefer to do this in distance, you could run at a track, or mark out a run ahead of time, and run 200 meters, then walk 200 meters and do 5 laps. It doesn't really matter what you base your intervals on, just that the running portion makes you work harder than you normally do. By the end of the running portion, you want to be going hard enough that even though you could talk, you probably wouldn't want to. Then, use your recovery time to bring your heartrate back down.
By practicing speed, you will be ready for the moments in the race when you feel like going faster. They won't tire you out completely, because your body has done them before. And, a surprising thing might happen, you just might end up running faster overall!!