After starting this post, I researched some more and actually found the frostbite table.

At those temps, assuming a 15mph ride, it would actually take about 30 minutes for frostbite to set in. So, a couple miles on the bike - while unpleasant - may not be dangerous to the skin or extremities...but as for me, I wouldn't have done it when I was 20 (and the top of my head was naturally insulated then!)