The brain-body mechanism for pacing is discussed in this article. If you’ve never abandoned your watch and tried running, you should. Not just one run. It takes weeks. You’ll eventually start running by the way you feel, and not by what your watch is telling you. Try it! But you have to give it time! Not one or two runs because remember, each day you head out your body is in a different period of recovery or breakdown. In other words, there will be some days when you’re going hard and days when you aren’t. Once you get accustomed to running without the watch, when you put it back on you won’t believe the pace you’re running and how you feel!
My guess: The runners were relying too heavily on high goal-setting, heart-rate monitors, GPS systems, equivalent-performance tables, and the like. In other words, they used one or several of these tools to pick a goal pace faster than their actual race-day fitness. That’s what we high-success, high-determination, Type A runners often do.
Source: How To Run Your Best Marathon Pace—Guaranteed | Runner’s World