M.M.
Why not get a space heater for the garage? We have them all through the house (they're pretty safe these days, look at Vornados), and while you warm up - leave your sweats on - and let the garage warm up too.
It tends to work pretty quickly with one of those, so when you're ready to strip down to your workout gear, the garage won't suck.
Just remember...it takes 2 weeks to form a habit. If you can make it for 2 weeks, you'll be good.