Keep doing it.
It's relatively flat here in MN. So a few years ago I picked up a super-cheap Nashbar 29'er single speed with no suspension and disk brakes. I outfitted it with studded 55mm tires and it is my winter ride. Saturday it got up into the 20's so I did a 23 mile ride that left me exhausted at the end. Pushing those wide, knobby, studded tires through fresh snow really wiped me out. I do a lot of riding on the trainer inside which helps a lot but only steep hills compare with cold air and a surface like oatmeal! After a winter of riding that bike, I want to tell you I feel invincible the first time I take the road bike out in the spring!