Running an optimum marathon requires four things:

1) An accurate and realistic assessment of a target time. This is absolutely critical. Be  too optimistic with your goal and even a starting pace that you think is a little conservative will really be too fast and will result in a lot of lost time as you slow in the latter miles. OTOH, set a very conservative goal time and you will lose more time in the first half than can be overcome in the second half. If hot and/or humid weather is expected during the race, it is necessary to adjust the realistic goal accordingly.

2) A "smart" race plan that is based on pacing that is neither too fast nor too slow for the first half. A variation of more than 10 sec/mile either way from your realistic goal is too much. More than that too fast and you face a high probability of a late race crash. More than that too slow and the result is the same as with an overly conservative goal....you lose more time in the first half than you can make up in the second half. However, 5-10 sec/mile slower than "ideal" pace is better than the same amount too fast because it builds in a fudge factor in the event your goal time/pace really is a bit optimistic.

3) Factor course terrain variations and any significant temperature rise that is forecasted to occur during of your race into your pacing plan. Although we talk about running even or slightly negative "splits" as being desirable, the real objective should be to run even or slightly negative "effort". That doesn't mean that mile 26 should feel like the first mile did. It means that uphills should be slower and downhills faster. And that if you experience a 20 degree temperature rise during your race that puts you into "warm" or "hot" marathon conditions, then the latter miles should be slower than they would be under constant temperatures. Factoring in these variables can make an even effort race actually produce positive or negative time splits. For instance, I consider the downhill Steamtown Marathon to be a somewhat positive time split course when run ideally.

4) The discipline to control race pace and not let your freshness following your taper and the excitement and adrenalin flow of race day seduce you into deviating from your plan and going out too fast.....or making a last minute change to your goal and race plan. I always like to say that you should make your last thought before the start of a marathon: "If I'm not worried that I'm running a little too slow in the first half, then I'm probably running too fast."

Jim2