You know, I used to have a similiar problem on long rides. Right around mile 60 or 65, I'd just lose all energy and it felt like just keeping the pedals moving was a massive effort. I was with my husband for all of those rides and each time, he'd keep me going. Usually about 15 miles later, my energy would return and I'd feel fine. I have no idea if it was nutrition or what and I never really studied it much because I just knew that if I kept pushing, I'd get past it. It happened on every ride over 70 miles or so.

The funny thing is, my longest ride this year was at Crater Lake. We did ~78 miles and I never hit that bad spot at all. I was tired and my saddle was killing me, but I never lost all my energy. The main difference between that ride and others I've done was both what I ate and when. For the Crater Lake Century, I ate at every rest stop and they were really well placed for my stomach. I swear my tummy would grumble and I would think 'hmmm, I'm hungry again' and BAM, there was the next rest stop. Because I was in the midst of a weight loss challenge, I was careful with what I ate, too. I had my typical protien oatmeal prior to the ride (same thing I always eat before long rides/races) and then at each rest stop I would focus on fruit (mostly grapes) and sandwiches. They had 3" subway sandwiches so all told, I think I ate 3 or 4 of those (turkey) over the course of the day. I also had a home baked cookie at each stop, too. I did not touch my own food (sharkies and cliff bars) and my water bottles were filled with water in one and with Nunn in the other. I avoided all crap (like store bought cookies, trail mix, donuts, chocolate, granola bars, etc) and it really worked well for me.

Anyway, I think that for me, consistent fueling of QUALITY food (which in my case means fruit/grain carbs and protein, not sugar) was what made the difference.