I assume it wasn't easy to stick to the diet during the ride? Did you just not worry and make a decision to abandon the diet for the duration of the ride and pick it up again when you got home?
We didn't even really try. Once you're riding for 24 hours you'll use every carb you swallow.
Exactly this.
When you eat carbs the body releases Insulin to store
unused carbs/sugars. You only carry about 2,500kcals of carbs in your blood/liver as "easy access" fuel and, assuming you're burning about 500kcals an hour on an Audax, you'll struggle to eat (and metabolise) that many calories, at that rate, for 24 hours. If the carbs are gone then one of two things can happen...you can Bonk..or your body will start burning fat reserves (ketosis).
Example - Unfit cyclists (see typical London-Brighton charity rider) are probably burning 800kcals an hour or more. They aren't adapted to burn fat so they run-dry of carbs/sugars after 2-3 hours and bonk big-style. That's probably about 40 miles into L2B. A typical fat-adapted Audax rider wouldn't need to eat, or bonk, on a 50 mile ride in summer.
I think most Audaxers experience a slow, steady, improvement in the way they can metabolise body fat for fuel over time. It may start off, in the first year, that you bonk badly during a 100mile/200k ride. I know I did, and I was constantly hungry. A life of carbs means you aren't "fat adapted". It hurts when you try and access fat. After a few years i could get round a 200k with a bowl of soup half way round. I'm fairly sure I could do a summer 200km on water alone if I kept my heart rate down and didn't try to burn fat at impossible rates. (I specifically say Summer because cold weather and wind-chill makes huge calorie demands on your system).
Over time your body becomes adapted to burn fat as fuel, it's a natural state for most mammals, that's what fat is for, it's your energy reserve for lean times.
A ketogenic diet (or fasting) can train your body to switch to fat burning. Some people find it extremely hard but, as an Audaxer, I think i was well conditioned for ketosis.
Ketones are a more efficient fuel for your brain, that's why people experience a clarity of thought in ketosis. It's assumed that ketosis is evolution's way of making you more efficient at locating your next meal. You think better, you move better, you catch Antelopes better. Why would evolution make you worse at catching Antelopes when you were hungry?
So Ketosis isn't a fad, it's a natural state for us to be in. Think what our eating patterns were when we all lived on the plains of Africa, as hunter-gathers. Hungry...hungry...hungry...hungry... ANTELOPE FOR DINNER!!!!