Here's incentive: If you think skiing is already fun, it will only increase in fun-ness the more fit you become!
That's my motivation right there! My first 2 years skiing, I was in decent shape because I was involved with Team in Training as a cycling coach and trained for and did 2 centuries a year. Then I stopped cycling (lost my job, enrolled in college, then found another job which meant I was working full-time and doing coursework) because I didn't have time. The third season skiing was "eh" but last year was downright awful, depressing. I was way out of shape, a lot heavier than I'd been the first 2 years of skiing--not to mention the knee surgery I had in in September 2006, and it hurt like hell to ski. I spent more time pouting in the lodge than I did on skis last year.
I made the decision at the end of ski season last year that it was the last time I'd be out of shape for skiing. I haven't been 100% regular with working out, but I've done enough of it to have prepared me much better for this season, and I'm having fun again.
I do find that each time I ski, I get a little bit stronger. I try to target doing 1 more ski run each time I go (or try to ski 15 to 30 minutes longer than last time).
At the gym, if I'm pressed for time, I'll do some stretching and a 30-minute hill workout on the elliptical machine. I find that kind of workout is kind to my knees, but really gets my heart rate up and gives my legs a run for their money. I alternate upping the resistance to simulate going up the hill with a minute or so of lower-resistance "pedaling" but without holding on, so I work the smaller stabilizing muscles in my legs. Otherwise I'll do that, plus some weight training. It has made a huge difference this year!
Am I tired when I get home? That's an understatement! If I can stay awake during the car ride home, I usually collapse on the sofa once I get home and take all my wet stuff apart to dry. Hey, my body worked hard, and it's tired!
Simone, regarding your calf pain, the first thing I thought of was tight Achilles tendons and calf muscles, because the first few times I skied, mine actually felt bruised they were so sore. Now I stretch 'em every morning and evening while I'm brushing my teeth.