I was in almost the same situation as you when I started commuting in September. I hadn't been riding regularly for a few years. I'm in pretty good shape, but not an athlete. My commute is about 6 miles (12 r/t). I didn't have to work up to it at all in terms of fitness. It was more a question of gaining confidence riding with traffic and really getting to know my route.

Regarding MacNasty's estimate of how long it will take: I think that depends on your route and riding style. I have lots of places where I stop for lights and traffic. Plus, I ride wearing professional clothing, so I'm going slowly enough that I don't arrive at work drenched in sweat. 6 miles takes me just under 30 minutes at that pace; that's total time, including stops for lights and traffic.