As Shimoda said above darkness and cold (-15C at the moment) is going to be your main problem. Right now at 1 PM when the sun is (almost) shining you could get away with 100 ASA film but two-three hours later 1600 ASA is barely enough. Maybe you can squeeze in a short photography session during your lunch hour? It's usually bright enough to snap a few photos then. Another option would be to carry a small tripod.