You could always try some carb loading on the days leading up to your long rides. I'm sure you're in great shape, but your body is definitely trying to tell you something. Do you try to ride at the same pace for 70 miles that you do at 35 miles? Have you tried backing down your pace a bit for the longer rides so you don't burn out more energy than you are replenishing?
“Hope is the thing with feathers, that perches in the soul, and sings the tune without words, and never stops at all.”
- Emily Dickinson