I’ve discovered through personal experience that if i do a few day dry fast about a week before an ultra run I don’t seem to get tired or burned out on a run, I also don’t get sore after the run. My theory is that toxins that are stored in the glycogen in your muscles is used up and those toxins are released prior to the run rather than depleting them during the run, releasing the toxins and creating pain, inflammation and soreness. The fast creates fresh and pure glycogen/energy to be used during a long run.
Just my experience/thoughts, hoping to hear if anyone has something to add
I think it might be due to increased accumulation of osmolytes during your fast.