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>When I run for more than 90 minutes without taking in carbs, my body goes into a ketogenic state very similar to a multi-day fast.

That seems rather implausible. Do you actually test your ketones in both conditions, or is this just some subjective assessment?

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That is the normal state of ultra endurance athletes during long runs[0]. A lot of training stimulus is fat adaptation. Coaches will tell you that you begin to exhaust your glycogen stores beyond the 90 minute mark, and I can tell you subjectively that the body switches over around then. I have done fasted runs up to 3 hours before. Runs of that duration are physically impossible without burning fat as fuel.

[0]: https://www.tandfonline.com/doi/abs/10.1080/02640414.2019.17...

The full text of your link shows the athletes didn't have elevated ketones until the second day of the race. After 80km, or about 12 hours of running, on an energy deficit.

I took this a subjective feeling of the experience, but I agree with you. It is almost literally impossible (some high level athletes can accomplish it, I believe, but not after 90 minutes) to actually enter a ketogenic state from exercise.

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