My commute is about the same distance at -23c as it is in warmer temps
(sorry, couldn't resist)
My commute is 11.4 to 12.5km (~7 - 8 miles) each way. I'm pathetically slow in these conditions - typically takes me about 45 minutes each way. The temp really isn't a problem - If anything I sometimes get a bit to warm (toes excepted, soon to be fixed)