-7C? I understand. I've had several of those commutes so far, this year. Coldest so far, has been -11C. One day, I wimped out, and didn't ride, as it was -14, but had a 19mph headwind. I would have ridden, if not for that wind. I guess you just get used to it after awhile. My biggest problem in those temps is keeping my hands warm. A balaclava really helps keep your face and neck warm. I ride in my outdoor work clothes, so I'm not concerned with how I look. In really cold weather, I've even worn one of those paper dust masks. If you're not breathing that cold air, your whole body will feel warmer.
Thankfully, in this weather, my commute isn't nearly as long as yours! I never can ride fast in cold, due to heavier clothes, usually windy, and I'm often riding with the studded tyres. I'm impressed that you do you fastest times in cold weather! And I DREAD the thought of repairing a puncture in those temps! Been lucky, so far!
This December has been unusually cold for us. Usually, we get a few cold days, and a few warm days. This December, it's been consistently cold...more like our typical January. Well, maybe the rest of the winter will be warmer than normal!
Hang in there! When those warm days arrive, you'll realize that you've taken the worst that winter can dish out! Gives you a "warm" feeling!