i should have been more clear in my original post. going downhill doesn't mean bonking. Going downhill means I could no longer sustain the intensity of the first two hours, that intensity being 75% of threshold power. I'm sure I could go for 3 hours at 60-65%, but not at 75%. As for actual caloric deficit in those first 1.5 hours, it's about 1200-1300 kJ of energy burned, or ~1200-1300 kCal.
While one may burn only 500-700 kCal, sharp anaerobic efforts (like those in MTB) tend to deplete that storage at a faster rate