I wonder what the mechanism is. Could dehydration have anything to do with it?
I don't think so. I was very careful with hydration and morning water content on my scale was consistent with the last five years or so I've been using it.
I think it is more likely the work level led to a change in other aspects of the blood so that it could carry fuel to my legs more efficiently and that in turn interacted with other aspects of the blood.
Heart rate was very low, most notably on climbs so there was clearly improvement in fuel delivery. I have done any research yet, but it warrants some exploration - and I need some data points if I'm going to tackle any real endurance efforts.