I divide my winter exercise activities between fitness walking, biking, and snowshoeing- depending on the weather, snow, and road conditions.

I do find that it is the most difficult to keep warm when biking (self-made wind chill, more exposed hands and feet that can't move around enough to keep circulation going...). So, when you look at the thermometer and it's just too cold to bike....it may not be too cold at all to do a brisk walk or jog and be able to keep perfectly comfy. My hands and feet particularly stay warmer when walking in the cold than when biking.
I can be perfectly warm doing a 5 mile fitness walk in 10 F, -when I'd be totally freezing my butt off on my bike.