When energy starts well and drops off

I had one of those days yesterday. After a taper week, I did a hilly 6 hour ride. The first two hours were great. I felt great and power was good. However, after 2 hours I could feel my energy dropping off and not feeling so good.

I still need to improve my nutrition plan, especially for the hilly days because they take a lot more from me.  The glycogen depletes quicker on those hilly rides.  I expect I will end up mostly relying on on gels.  I am going to need a big stash of those.   

Low glycogen = a less pleasant ride, low energy and more suffering.  It will delay recovery too. I can't afford that to happen when it comes to the main event.