Pete:
Great question. It is more likely that low glycogen caused your perceived fatigue. Doing as much vert as you did presumably with an alpine weighted pack means that you you were doing a lot of strength endurance work that day. This type of work will gobble up glycogen to fuel those fast twitch fibers need to carry that weight up the approach and climb. When your glycogen reserves get low the ability of those same FT fibers to keep cranking our the power diminishes and those fibers “fatigue” due to fuel depletion. So the FT fibers drop out of the game and your ST fibers end up being the only ones being used due to their much high endurance. Since these ST fibers have adequate endurance you won’t feel you legs as the limiter any more. This is why the ME workouts we describe need to be done in a non fatigued, well fueled state so that those FT fibers which are your endurance limitation get the training they need to become, Remember: If you can’t recruit the fibers you can’t train them.
Scott