I've run when it's 1°F, 0°F and even, perhaps, a few degrees colder, but this morning, even though I left it until 7am to get out of bed, when faced with this:
(that's -24°C) I decided that an indoor day might be in order.
So I spent the day watching TV, internet videos, reading (I'm about halfway through the excellent Wool by Hugh Howey) with a bit of cooking and clearing up. I've also been playing Halo 4 - yes I'm this guy:
but with nearly up-to-date hardware and a more manageable 1 year delay.