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...