I did the 50 mi club ride to Boca Raton today. I was ride leader, but I bonked at about 38 mi on the ride home and did the last 12 mi solo. Bonked on the same ride last week at about 35 mi.
I'm on a low calorie high fat diet and while I'm in Ketosis, I'm still 2-3 weeks from being fully keto-adapted.
On thing I did notice was the effect of fat metabolism being much less oxygen intense than carbohydrate metabolism. On the ride home though the park we were doing 24-25 and I looked and my HR was 88 - 92% max, but I was hardly breathing. Normally at that effort level I would be sucking wind bad.
Once I'm fully keto-adapted I should be able to maintain a high level of effort for extended periods. I want to be ready for some grueling rides in upstate NY in the mountains. It's a real change from the billiard top riding I do here.