At -45C/-48F with the windchill factor, I didn't even THINK about running outside today.
Treadmill. Just set it at 9:10/mile and kept it steady for 110 minutes.
Gotta remember to turn off the little heater I have running in the laundry room. I tripped the breaker while running, which turned off the treadmill, which almost threw me over the front bars and into the fish tank.