Honestly? I am still outside for an hour and Rocky is 12. we don't cover the same distance as his age but I am absolutely still out there with him twice a day (20 mins in the morning; 45-60 minutes after work). If you can't have someone else help out (dog walker, doggy daycayre) or find indoor play dates then you'll have to bite the bullet and go out. SO MANY fun things to do in the winter. Days are getting longer so we don't have to stick to sidewalks anymore either.
Last winter i'd go "cross country skiing" in the field with penny. or snowshoeing. she'd hop around in the snow. I still run outside in the winter (yes i live in Canada and it's cold) even without a dog partner. Rocky prefers trail walks so we try to hit those as much as we can (plus it's protected from the wind there).
Are you able to shift your schedule some so you can either do morning or evening walks if you are concerned about the dark/safety? (with days getting longer this gets more doable).
Find a friend to walk with you? fellow dog walker? Set-up play dates in friend's yards/homes (I did this when it's icy - we'd do indoor play dates).
Take a class? I am taking scenting with Rocky.
Train more in home? make him work for all his meals (training, new tricks, interactive games)? Some classes you can take online (like scenting).