The roads leading to my traills are narrow, windy, and untended in the winter and I can't really justify wrecking a $20,000 truck to get to the trailhead. Riding on slick, curvy, icy roads to get to the trailheads is asking to be killed. The trails for the most part run over steep mountains or along high ridge lines, and a fall off a highwall would be fatal if you were lucky.
I usually get back to the trails in april and I can ride until late october to early november. I stick strictly to the gym, as it is better for my well being in more than one way. I will check back and see if anybody has any biking specific exercises.
ETA: a quick glance through the yellow pages and no kickboxing academies are in my area.