Hi,
Not run a marathon (yet) but have skiied in training for races up to Half. How experienced a skiier (or boarder) are you? If you're a beginner you'll be wasting a lot of muscle effort to learn, if you're experienced then it will depend if you're coasting down motorway blues or giving yourself a workout on a mogul field as to how much you'll suffer.
If you don't take it easy, you could expect some DOMS which will have gone by the race, but is probably not helpful in training.There's the question of "specificity" - training your muscles for the task at hand, but I wouldn't have though a week would make much odds, especially if you do a bit of treadmill whilst you're there to remind your legs what their day job is.
That just leaves the prospect of getting injured, which could happen anyway, and of course the apres ski...
On the plus side, I found that the level of fitness running gave me made a big difference to my skiing. I going skiing with a ski race nut who was staggered at my new endurance (if not my technique).