Fading in the final miles is common in the marathon. It can happen anywhere past the half-way point, and is typically a combination of mental and physical fatigue.
The typical marathoner will slow down 14% in the last half of the race. To put this in perspective, for a 4 hour marathon, this means 1:52 in the first half and 2:08 in the last half. For a 5 hour marathon, this would be 2:20 in the first half and 2:40 in the last half.
