Maybe you think you are in good shape. 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. If you have access to a health club, an exercise bicycle or stairmaster are the two best pieces of equipment you can use. 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 stadiurn or high-rise is another option. Step aerobic classes provide similar beneflts, and can be done with ankle weights to further improve the workout.