Cross country skiing is a great workout that is much easier on the body than say jogging or other sports that jar the body. The movement of the arms and legs forward and backward lead to a high level of cardio impact without necessarily making any hard moves on the body. It is a fluid motion that maintains a level of resistance for muscle building.
Improving one's body means getting out and putting a level of stress on it. That doesn't mean we should wear it out and cause injury. It does mean we find activities that are physically challenging while still being enjoyable. Playing in the snow is enjoyable to me!!!