This is what Hunter and Coggan say about fatigue:
20min 60min 90min
Below Average(Strong Initial Effort) 100% 7 to 11% 15 to 25%
Average 100% 4 to 6% 8 to 14%
Above Average(Strong Endurance) 100% 2 to 4% 5 to 7%
The formating got messed up, you can see it here
Two hours are not in there, but it should help you guess.
I would try and figure out what exactly causes the drop off and then try and fix that.
A WAG is that your glycogen stores are depleted. If that is truly the reason, you could tackle your feeding strategies and try and move the enegery source % to your favour, meaning that you use more fat and glycogen for the same effort by focusing more on Endurance and Tempo levels.