I grew up in St. Louis so I got used to riding in cold weather, cold rain, snow, and silly cold temps. (once rode to work at neg. 6 Deg. F).
I did what Machka advised above, essentially, wore clothes to suit the weather. Not always "cycling clothes" though I did move to more specific cycling clothing as I got older.
Then, when I was 45 I moved to Southern California (1995) where they don't have winter weather as I knew it in Missouri. Or, well, they do if you want to go up into the mountains to seek it out (which I do sometimes, crazy as that must sound).
Anyway, maintaining fitness wise, do whatever it takes but get out there and ride. Yes, I know sometimes it's hard to find motivation on a miserable cold grey day that's accented with freezing drizzle, but once you get out there and conquer it, you will feel so much better, not only physically, but also in terms of self-fullfillment, i.e. You actually went out and got in some miles, even though the conditions were miserable.
Just remember, the weather is not as bad as it looks from inside your warm dry house. Once I'm up to temperature (takes about 10 minutes for me), no matter how bad it really is, I can convince myself "It's not that bad!"
Rick / OCRR