It's dark and cold after work at this time of year, so I have to force myself go and do some fitness training! Running is out of the question because it's just too high impact, so I'm out on the mountain bike a lot. During the week it's all road riding (albeit up the steepest hills I can find), but I managed 100 miles last week and I'm aiming to ride at least 50 miles a week until the Spring.
On the weekends when I can fit a decent ride in during the day, I ride about 20 miles and do as much of it off-road as possible.
There's no substitute for bike time, but getting out on the mountain bike is about as close as it gets.
I'll go and ride at some trail centres soon, as long as it's not icy...