PACING IS VERY IMPORTANT.
Regardless, or in spite of the amount of training you have done pacing a race correctly is of paramount importance. You receive your time and position at the end of the run not after a 1/4 or 1/2 way through. In the London Marathon this year(2012) I heard one man turn to his friend at 3 miles and say "I don`t think I can keep this pace up until the end". I just shook my head and hoped he would be ok.
The amount of people who run off too fast at the start never ceases to amaze me, and this is people of all standards, novice and experienced alike. When I ran the Dublin Marathon, along with 8000 others, not one person overtook me after 10miles. It was a great feeling to constantly go passed other runners.
Physiologically it makes sense to pace the run as well, as we don`t want to burn up all our energy reserves early on. Also, if we set off too fast we will end up with lactic acid in our legs and the recovery will take longer than the initial time gained. The result is we end up running slower than we could and should have. Try to keep an average effort throughout the race, only trying to pick it up close to the end, when you know that you can carry on at the quicker pace until the end. Trying to pick the pace up in a marathon is obviously very difficult..
When we go out for a long training run we don`t generally set off a fast pace , so why do it in a race. Equal effort( even on hills) is the most efficient way to get round a course, it allows us to slow down on the up hills and take advantage of the downhills. It stops us getting unnecessarily out of breath.
LEARN TO PACE YOURSELF