Great question and perfect example of the complex interactions of training effects. ALL of your aerobic training has some ME effect. After all, it is the muscular work that moves you. It is good to think of ski touring, mountain running, mountaineering etc along a spectrum of strength. A low angle tour on previous skin track or a mellow run on a flatish trail are going to have a much lower strength requirement. They are going to recruit fewer muscle fibers, and especially for the reasonable fit, many fewer FT fibers, so these workouts will demand less in terms of muscular endurance because those ST fibers needed to get you up the trail are very well endurance trained.
Now: Steepen the grade, add some pack or boot/ski weight and toss in some trail breaking for good measure and you move toward the other end of the strength scale. You need to recruit more muscle fibers. When the going is tough you may be needing to call upon some FT fibers, which by definition do not has such good endurance as the ST fibers you used earlier. Now this ski tour or run or climb has become much more of a muscular endurance workout.
Our prescription for ME workouts https://uphillathlete.com/vertical-beast-mode-what-is-muscular-endurance-why-it-is-important-for-any-alpinist-or-mountaineer-and-how-do-you-train-it/
is designed to push you WAAAAY far to the strength end of the spectrum by using a heavy pack and very steep hill so you notice that the muscles are the limitation.
What you are experiencing is typical when the work starts to move toward the strength end of the scale. Your legs don’t recover fully from day to day and fatigue begins to accumulate. You are in the grey middle area of the spectrum. Not easy but not a full on ME workout.
What to do? You are wise to wonder how much is too much. That’s a very individual question with no answer that works for everyone. Folks with more leg strength and ME will be able to handle more back to back days of this. Pay attention to your fatigue: backing off when you notice a significant increase or after a big jump in training load like you mention and these workouts are great aerobic base training even with their high muscular load. BUT…You need to be mindful of that fatigue and allow adequate recovery somewhere along the training cycle. Will that be after one big day or will be after 3 no one can say but you.
Things like this great question is why Steve and I went to such lengths in our book and continue to do so on this website to explain theory behind the training so that you and other mountain athletes can make these kinds of informed decisions. It is not black and white. There is no one size fit all formula. We want to provide an intellectual framework within which you can think about your training.