ILoveSkiing, it sounds like you are trying to fight muscle fatigue by doing pre-season training. There's another better way to stop the fatigue. Last year was your first year skiing and your legs were killing you. That's not lack of strength. It's a result of skiing in a sitting-down position, holding your entire body up with your thighs. Doing sustained squats all day long will do that to anybody. The best solution is to stop doing squats, not to build up your quads so they can keep doing that without fatigue.
So how to stop skiing in a sitting-down position? You are new to skiing; this stance is quite common among new skiers. It's so common it's got several names; the most common is "skiing in the back seat" or "sitting back." Also known as simply skiing "aft." You need to get forward, or centered, rather than aft. The very best way to do that is to take a lesson and have an instructor watch you and tell you how to get out of the back seat. You need the experienced eyes of a seasoned instructor right there beside you to act as your coach; people tend to think they are forward or centered when they are not.
So my suggestion is to take a lesson first day back on snow. There are more benefits to learning this than less fatigue; you'll be more in control of the direction your skis are going and will have more control on steeper pitches once you stop skiing aft. You'll be able to use the fronts of your skis because they will be pressed down into the snow instead of gliding along on top of it, unweighted. You'll know you've succeeded when your muscles stop faililng; but that won't be a good indicator if you just get strong enough to keep squatting all day. I'm not saying getting strong isn't good. I'm saying that technique is probably a big player in your leg burn.
That is for sure a contributor, I was skiing in a back seat position as you describe for the better part of my ski season last year. Then I had instructions and I got better at it, and corrected it almost completely, alltho not perfectly as sometimes I still fell back a bit.