Rich666, below I've copied and pasted one of my posts from a previous thread. Perhaps something in there may be useful.
"There are some great points for discussion here. IMO, which seems to be supported by several recent threads, the recreational skier is looking to ski longer without getting tired and developing burning muscles - i.e., endurance. Leaving aside, for purposes of this issue, the obvious fact that the most common cause of excessive exhaustion is poor skiing technique, it's worth looking at what best develops endurance and why. So, what does exercise physiology tell us about endurance, how in the real world do elite endurance athletes train and, finally, are these data pertinent to non-elite, recreational athletes?
Without going into the nuances (and hopefully limiting jargon as much as possible), there are two major determinants of exercise capacity: VO2 max and lactate threshold. VO2 max is a measure of the capacity of one's cardiocirculatory system to deliver oxygen to tissues; lactate threshold represents that intensity of activity above which levels of lactate begin to accumulate rapidly in the blood because the body's ability to clear it has been exceeded. Both are trainable -i.e., can be improved with appropriate training programs - lactate threshold much more so than VO2 max. Both are involved in our subjective perceptions of how hard we are working -i.e., how unpleasant any given level of activity seems to us. But, each responds primarily to a different set of training stresses. VO2 max responds to periodic, high intensity exertion which, by definition, has to be limited in duration. Training effects plateau relatively rapidly; indeed are probably largely genetically determined for each individual. Lactate threshold responds to training volume, basically the more the better, and is capable of showing very great improvement over long periods of time (seasons or even decades) if training is continued. It doesn't respond to intensity above a certain threshold, it responds to duration. We have a very good understanding of why this is so down to the molecular level.
What do elite endurance athletes do? They train a lot! One doesn't need to have a detailed understanding of exercise physiology to be a good coach, but one's methods must produce results or Darwinism goes to work. Athletes at the top level in running, cycling, rowing, cross-country skiing, etc typically train from 800 (at the low end) to 1500 hours per year. Of this, approximately 80% is conducted at 60-65% of VO2 max (below the lactate threshold) and the balance is high intensity, periodic training at above 85% of VO2 max. Numerous studies, including cross-over trials, have shown that this is a good mix and it is consistent with the physiological considerations above.
There are few controlled studies on non-elite, recreational athletes. Part of the reason is that it's difficult to get people to train easy, easy enough (and long enough) and to train hard, hard enough! Consequently training intensity tends to converge in the middle, having neither the intensity to push VO2 max, nor the volume to make big gains in lactate threshold measures. Recently a team in Norway succeeded in getting three groups of amateurs through a study that confirmed that the 80/20 split produced the best gains in endurance even at a total training volume of just 10 hours per week (results not yet published).
So, the data support the idea that, for average people looking to ski with more endurance, it is more important to train lactate threshold than VO2 max, to train longer and easier, and to limit high intensity periodic workouts to 20% of total training hours."