The only time I hallucinated was after a single-day double crossing of the Grand Canyon, a 15-hour run/hike iirc. I would see a random tetris game in front of me even though I never played.
It feels like a wakeful dream. Where despite having open eyes and even interacting with the world on some level, the brain is off watching an entirely different movie.
Interestingly, I’ve done a few similar hikes and had plenty of energy after to cook a big pot of chili at our basecamp and enjoy a couple of beers. Exhaustion is weird.