I vote with the folks who are mentioning ponchos/capes. I bought one from Oregon's Center for Appropriate Transport and have been using it successfully for several months.

My commute is nowhere near as long as yours, but I do ride, rain or sun.

The poncho protects me from the rain coming from above, and keeps me cool(er) and less sweaty than if I enclosed myself in traditional rain gear. There's a constant flow of air from below, and the fenders on my bike keep the splash-up and road grime off. My feet will get a bit wet from puddle splash-up, but other than that I stay relatively dry except in strongly directional rain.

If you go the poncho route, make sure you have fenders on your bike, and choose one that is either bright or reflective.