I am an advanced beginner (can do turns with parallel skis, just now learning about weight transfer to carve turns). The first 3-4 times I went on ski trips, my quads would tire easily. After only 1-2 runs, I can never make it down a single run without stopping to rest my legs SEVERAL times. Granted, my legs are not in great shape, and I did not train much for those trips.
I just got back from a trip to Whistler, and for about 2 months before, I did some daily work trying to upgrade my leg stamina (wall sits - worked up to 20 minutes, squats and lateral squats on a Bosu, and static squats on a Bosu - up to 20 minutes). I got on the slopes at Whistler, and my legs felt as if I never did any training! I was tired very quickly, just like before. Granted, my training regimen was not very long or very intense, but it was way more than I have done before, so I would have expected SOME improvement.
On the other hand, my fiancee doesn't exercise hardly at all, but never has tired or sore legs when skiing. That's frustrating to me. Granted, she learned to ski when she was a child (I started when I was 30, and I'm 32 now). So, I'm sure my technique also has a lot to do with it.
Do some people just never get the leg stamina without a really intense training program? I only ski 2-3 times a year, so it's hard to motivate myself to train only for the purpose of skiing for many months on end. I'm sure a cross-training program would help. Any suggestions for efficient ways to increase leg stamina? I was thinking about doing some mountain biking and strenuous hiking as part of my cross-training. Any particular indoor exercises you would suggest?