Spinning classes. Sign up for an intermediate to difficult spinning class at your local gym once or twice a week. Combine it with a core work class and some weight lifting.

Ride outside whenever you can throughout the winter.

Do a bit of cross-country skiing and snowshoeing.

And on the remaining days, hop on the trainer for 30-60 minutes, doing some base mileage or commercial intervals.

By spring, you should be in pretty decent shape.