When I was a marathon runner 1/4 century ago I became very familiar with the rule of thumb - 2 hrs of glycogen. I could run 2 hours of 6 minute miles followed by 40+ minutes of 7 minute miles. You could set your watch by my demise.
My current guesstimate of max HR at the advancing age of 67 is 165. I'm cruising at 120-130 and climb at 140 or so. On any reasonable downhill I'll just turn the pedals to keep things moving. 100K on the tandem take about 4.5 hours depending on climbing and while I'm working hard, I don't bonk. I may cramp, or feel the onset of cramps, but no feeling of hitting the wall.
Is this simply a question of not having the aerobic capacity to burn glycogen at that 2 hour rate?