What can I do to keep fit over the winter months?

I've been walking for exercise, at a good steady pace for 45 minutes just about every day after work. I really enjoyed doing this for exercise. Now the days are getting shorter and it's getting dark earlier, sometimes it's dark before I return back home. I don't like walking in the dark and soon it will be too snowy and cold to comfortably walk the way I've been walking. I don't want to go all winter with no exercise so what can I do for the winter months to replace walking?