How to get in shape for skiing? The snarky answer is to never get out of shape, dude. But that's not real world, especially at our age.
My practical answer is to start with walking. A few times a week.
Yea, but don't just walk forward. That would be too easy.
In order to protect your knees and build muscle strength along your entire leg, you need to walk backwards as well as sideways to the left and right, aka grapevines.
When I walk for skiing in a gym, I spend about five minutes on the treadmill walking forward, then slow the machine down, turn around, and walk backwards for five minutes.
Then I walk sideways. I turn sideways on the machine, hold on to the handle, cross one leg in front of the other as I step, then cross it behind as I step. In aerobics class, this is called a grapevine. Do it for five minutes in both directions.
Repeat for as long as you want.
Builds flexibility and strength in the knee and hip joints
Reduces the risk of an ACL injury because the muscle strength on each side of the leg becomes more equal. Those strengthened muscles do a better job of protecting the knee 360 degrees. You now no longer have a huge discrepancy in strength between your quadriceps and your glutes that cause your ACL to rip from unequal forces.
In that same vein, if you use an elliptical machine, don't just pedal forward on the machine, but reverse direction for an equal amount of time. When you pedal backwards, you mimic the muscle action that you should have when you ski: you are balanced on the ball of your foot and your knee bends toward your chest.
Another variation on the elliptical is to turn the pedals by raising up on the balls of your feet for a few minutes. Then try to keep each foot completely flat as the pedal turns. This helps strengthen the muscles in your feet, and helps stretch your Achilles tendon. Do this in reverse also.
Pretty simple, yes? Now get out there and give it a try.