Winter is a great time to slacken the amount of miles and time run and pursue other means of maintaining fitness. Swimming, cross-country skiing, snowshoeing, and alpine skiing are great ways to lessen the impact from running and switch it up. However, I have found the ultimate cross-training during these dark winter months to be Swiss-bobbing.
Equipped with small butt sleds known as "Swiss-Bobs", or as our versions are called "Merikan Missiles", we are able to get in a good uphill hike on one of our local ski hills and then come screaming down in 5 to 15 minutes.
When coming downhill at 25 to 30 miles an hour, please remember to wear a helmet and steer clear of trees, lift poles, warming huts, and other people. Be advised that screams leaving your mouth from pure joy not experienced since your youth are sure to happen.