I'm in good shape and work out regularly. But during those first couple of weeks on snow, my quadriceps burn unbearably. What's my problem?
Pre-season workouts are an important part of getting ready for the hill, but no matter how gym-fit you are, you will experience fatigue and soreness your first few days on skis. There is simply no way to artificially mimic the stresses of our sport. The way to be truly fit for skiing is to ski a lot.
Burning in the upper legs is part of the game. It signals that we are pressing the envelope and, as a result, getting stronger. But if you're well-conditioned, the burning dissipates almost immediately after you stop to rest.
The fact that your quads burn "unbearably" for many days suggests a larger¿but not uncommon¿problem. You are out of balance to the extent that you can't support your body weight on your skeleton. You rely too much on the muscles in your upper legs to hold you up and help you turn.
The problem may be technique. Skiing is less about brute force than about staying centered over your feet. I'll bet you try to ski in a stance that's too low (see Turning Points: Stand Tall, Start Strong). Constantly sitting back puts huge strain on the upper legs. That the pain goes away after two weeks is a testament to your strength.
Just as likely, there's something wrong with your setup. Something about your boots or the ramp angle of your bindings is encouraging imbalance. The boots may be too high, too stiff or ill-suited to you. The cuff cants and/or forward lean may be adjusted incorrectly. Imbalance problems can often be solved by using different wedging systems to adjust heel height.
Get checked out by a great boot-fitter or by an instructor or coach who is a balancing guru. Then, once you've modified your technique and gear, you may want to consider diversifying your leg workout. (Check out "Fit To Ski?")