Just posting to note my experience from yesterday. Did 96 miles, got up and had 2 slices of bread (140 calories) with some light jam on it and a coffee. On the bike I had a bottle with SIS beta fuel (340 calories) and another bottle with Nuun and during my ride used 3 SIS gels (3x90 calories) and stopped at a popular general store for a halftime cookie. So in total, I had maybe 1200 calories between breakfast and the ride. The ride itself was a steady endurance ride, 3355 kj of work as measured by my power meter (so about 3355 calories) with a normalized power of 208w (intensity factor of 0.67).
I really do wonder how much of the issues folks have with tiredness on long rides is from nutrition versus fitness and their ability to sustain efforts for extended periods of time?