I get up at 4:20 3 times a week and get in about 12 miles on the old mountain bike. I've been doing this for about a year now. Typically on the bike by 4:30 and get back about 6:10 in time for coffee, shower, and off to work.

If it's raining or below about 35°, I do some indoor exercise instead.

I find you need to be very vigilant in the early morning. Many commuters are half asleep and not expecting to see cyclists. Some don't even bother to stop at signs cause there is no one around. I always yield to cars even if they appear to be waiting for me. If I can't see them, I can't be sure they see me.