Short, easy runs teach you how to maintain a pace and finish in a consistent time. Tempo (faster) runs encourage you to push your pace and run a little harder than is comfortable in order to improve your speed. Long runs show you how to slow down your pace and conserve your energy so you don't run out of steam halfway through.
When race day comes with its fanfares and thrills, all that careful training sometimes flies out the window. The field is thronging around you; spectators are cheering wildly. You can hear your own heart beat. Even elite runners with years of training and experience can fall prey to race day jitters and pace problems.
When the starting gun cracks through your race day morning and the runners around you surge forward, remember above all else to pace yourself. Those guys breaking free from the pack at mile 2 may be on their knees by mile 24. Imagine yourself waving merrily as you pass them by.
You've trained hard, and you've trained well. On race day, all you have to do is trust your training and not let all the excitement get the best of you. There will be plenty of cause for celebration at the finish line.