Pacing: The only way to avoid the infamous “wall” is to pace correctly. If you go out too fast, no supplement, no goo, no over-distance running in previous weeks is going to save your butt. The answer is racing within your capabilities, and going like a bat out of heck at the end.
If the bulk of your training is on unmeasured roads or unmarked trails, the easiest and most practical method for measuring distance, checking pace and recording splits, is through the use of a GPS Time and Distance Watch.
During the marathon itself, it is a good idea to record all your splits. Unfortunately, mile and kilometer markers are often misplaced, missing, or missed because you were looking somewhere else as you passed.
Fear not! The 100 Lap GPS Time and Distance Watch can be programed to announce the approach of a split so it maybe recorded.
After the race is over, you can recall all your split times to be analyzed. With this information, you can see where you may have gone wrong (or right), and work on improving for the next race.