Not so sure an adaptation “trigger” would present in such an obvious and immediate manner; the body doesn’t change acutely.
I would think it reveals aerobic weakness (or strength), more than anything. If you’re good at 3hrs but really drift on a 5hr ride, that would be a clue of where you require more training.
Look at it like a very long interval. For example, if you always blow up near the end of a 3min VO2 interval, that doesn’t mean you’re getting stronger.
Then again, I’m not a physiology whiz so I could very well be off the mark.