Right now the running team that I coach is several weeks into training for their spring marathons and half marathons. Lots of them are planning ski weekends for Martin Luther King and President's Day weekends, and they're wondering how a weekend of skiing will affect their training schedule. Should they still do a long run that weekend? Or is a weekend of skiing enough activity for one weekend?
First of all, skiing, especially cross-country, or even snowboarding, can be excellent cross-training for running and is a great way to supplement your training for a half or full marathon. If you're training for one of those long distance events, you also know the importance of doing your long runs once a week and gradually building up your mileage. Of course, it can be beneficial (both physically and mentally) to have one "rest" weekend a month, where you either go down in mileage for your long run or do cross-training instead of running. So, many half-marathoners-in-training and marathoners-in-training can go away for a weekend of skiing and use it as a "down" week in mileage. However, if you're a little behind in your training, you may not be able to afford to miss a long run and may have to squeeze in a long run before you leave for your trip.
And if you're going to be away for a whole week of skiing, it's a good idea to try to do at least one or two shorter runs on a treadmill while you're away. Although you'll be active during the week, it's important to keep your body and mind focused on the event that you're training for.