Great question – this really gets to the root of the how and why with aerobic training for technical sports. As you’ve observed, improving a somewhat inefficient technique demands spending some time above your ideal aerobic zone, because those muscles you’re employing to ski correctly aren’t well-trained yet so they have to work harder. There are many different ways to approach this challenge but I’ll lay out a few:
– Drills: Using technique drills are a great way to isolate a certain component of the movement without making a “global” demand on the body and thereby increasing the intensity. For classic skiing, no-poles work is really valuable here. More specifically, you can use a flat or slightly-downhill grade and practice driving the rear ski forward and then “pulling” your hips onto that ski as it passes forward and in front of you, effectively “catching a ride” on that ski. This mimics well the powerful leg drive for classic skiing. Alternate legs and work on balance.
Another functional drill which also serves well as a strength-builder is using poles but keeping the legs still; you can use a striding motion with the arms on a flat or slightly-downhill grade, propelling yourself only with the upper body. Work on a full range-of-motion from the shoulders and think about “whipping” each arm from its rearmost position to its forward-most position, letting the thumb sweep low on the thigh as it passes forward. Creating a cadence with the arms in this manner can then supply a good rhythm for your striding when you add the legs.
– Speeds: In addition to using drills like the ones above, you can practice good technique through focused intensity or speedwork. I would recommend starting with shorter intervals so you can be sure of quality technique, and not let it devolve into an anything-goes-as-long-as-its-hard format. Try a handful of repeats of :20 – :30sec apiece, at an intensity which allows you to ski fast but without letting the technique break down. In this way you can train the upper limit of the muscles’ function but without exceeding their optimal range. Start with one session of these per week and as you improve, you can add a few in here and there to your normal aerobic sessions, but keep them limited and only do them when you can be sure that you can utilize a good technique.
Finally, it’s good to remember that as your technique improves it should be something which you can do at slow and fast speeds alike; I recall in training for skiing, hearing about Norwegian world cup athletes who would perform their long distance (3+ hr) workouts at very easy intensities, to the point where they were all but walking on longer or steeper climbs. And yet, to watch them you can clearly see that they’re still employing the same elements of good technique that they would at high, race-level speed: forward leg drive, engaged core, upright and forward body position.
I hope this helps; good luck!