You jog once in a while or swim 3 times a week. This does not mean you are in shape for skiing.
Skiing requires strength of the quadriceps, gluteal muscles and low back.
Because of the high altitude, above average aerobic conditioning is necessary. Both of these work the quadriceps and hip musculature as well as provide aerobic training. Of course outdoor bicycle riding is equally effective, and running stairs in a stadium or high-rise is another option. Step aerobic classes provide similar beneflts, and can be done with ankle weights to further improve the workout. If you are on a long ski trip or far from any exercise equipment, a number of simple exercises will help build your ski muscles.