Notes on doing speed work in -25 Celsius

So as week #3 of marathon training draws to a close, I can say for sure that doing at least some speed work in ultra cold temperatures is the way to go.  If I remember my biology classes correctly, most of the energy created by your muscles is, ultimately, just heat--less than 30% of the energy created is used for actual work.  Thus, the more your muscles work, the more heat you generate (a nice thing to know during a Canadian winter).
For example, on Wednesday, I slipped a pair of spikes onto my Saucony Guides and ran like a jackal on one of Saskatoon's many (and quite confusing) Crescent-shaped roads and down another street of the same name (*true) completing a 600 metre loop. Then I walked for two minutes and began the torture again.  (Another point I'm making is that even if the streets are sheeted in ice and the sidewalks alternate between clear and icy, there aren't any good reasons to ease up on training so long as you take the proper precautions.)  At any rate, the speed at which I ran was sufficient to generate more than enough heat to handle the cold--whereas running at my previous norm would be too slow to withstand temperatures colder than -15, to say nothing about -25 (never mind that the windchill was -33).