Well, here in the lower mainland we RIDE all year long! But I hear you, and suggest the following based on what I do. It actually is good to cross train other stuff rather than only riding. I snowshoe and hike, I have some great yoga for mountain bikers videos...and yes, yoga really is the shit! I has helped me immensely.
Rowing machine if you can find one! I also wear a FITBIT - it makes sure I get my 10,000 steps a day and is a good motivator in the dark winter.