I should say first off that I’m neither a ski instructor nor a fitness instructor and what I’ve got to say contradicts what other people have said, so treat it for what it’s worth.
I don’t think that beginning skiing presents a general fitness challenge of the order (say) of playing netball. Of course, the fitter you are, the better you’ll be at anything requiring co-ordination. And there are some specific issues that I’ll get to below. If you were a racer, acrobatic freestyler in the park or off-piste adventurer, things would be different. But, at this stage, in terms of strength and aerobic demands, I think you’ll meet more of them carrying your skis up the hill, side-stepping and (don’t forget this!) falling over and getting up, than from actually sliding over the snow.
Learning to ski is largely about maintaining balance and transferring weight smoothly. That’s a physical skill, obviously, but it’s not one that jogging or lifting weights will help you with, so far as I can see. I know some very fit people who are really terrible skiers! Sports that require co-ordination and force you to perform off balance are good, so I imagine that netball would be helpful (so too is soccer).
Two qualifications. First, I expect that you’ll have stiff muscles after your first day’s skiing, but I don’t think it would be less if you were generally fitter and stronger. Simply, skiing seems to use muscles that other activities don’t. Even when I was a lot aerobically fitter than I am now and played squash and soccer regularly, it happened to me. In fact, these days, I don’t have as much of a problem. I put it down to (1) having better technique – I spend less time “fighting” the hill, and (2) doing yoga, which seems to address muscles that running and weight-lifting don’t.
Second, there is one area where it’s very useful to have muscle strength, and that’s in the upper legs (quads/hamstrings). Even the beautifully groomed trails you will find in Colorado aren’t completely even, so part of skiing smoothly means adapting to the ups and downs. And having strong muscles in the legs is great for that. But you need a particular kind of muscle tone – the ability to absorb while being relaxed and adaptable, a kind of powerful springiness. Gymnasts have it more than weight-lifters – explosive strength isn’t any use if it comes with tension. Anyway, as a girl, I imagine that there is a limit to how big you’d like your thighs to be!
And finally, something important about health that isn’t about training. In my opinion, the biggest challenge you’ll face physically comes from the fact that you’ll be taking a long plane trip and coming up to over a mile high to sleep and skiing at up to 12,000 feet (sorry, I don’t have the Winter Park stats). That’s a LOT for your body to take: jet lag, altitude and dehydration all together. There’s a really good thread about altitude on the Fitness forum. As an amateur, all I can say is: go easy on the booze (especially on the plane coming over) keep hydrated and make sure you get enough sleep. And if you find yourself suddenly developing “flu-like symptoms” – headache, vomiting – watch out! It could be altitude sickness.
But enough of the negative. Go for it! You’ll have a great time.