I put in radiant heat (gas fired). The garage is 50 feet long, so the pipe and shroud are about 40 feet long (I left if 5 feet short on either end). Anyway, the heater is 100,000 BTU and works just great. I keep my garage warmer (about 15 deg C), as I have my treadmill and exercise equipment in it, and I'm using it all the time. I would highly recommend this type of system.
I agree with Fuzz though, the sprint from the garage to the house isn't great in this type of weather especially after you have been working out for an hour and you are soaked with sweat.