Catrin - when you say x-country skiing, are you referring to classic style? If yes, I agree with Crankin that you likely won't have knee issues above and beyond those found with cycling/hiking (though the descents can be tougher on the knees).

If, however, you are referring to skate skiing, I would comment that the torsional strain on the knee is greater than classical skiing, and I do get achy knees after a full day of skating.

SheFly