Happy birthday and GOOD for you! You seem to have the perfect setup to start bike-commuting--manageable distance, plus lockers, showers, rack at work--what else could you ask for? Oh yeah, a bike.

As someone who did NOT start out on a hybrid, I'd suggest seeing if you can find a touring bike to start on. Touring bikes are road bikes, not hybrids, but they generally offer greater durability and comfort than road bikes that are built more for speed. The glitch is that most bike manufacturers offer lots of fast bikes and hybrid bikes and mountain bikes, but usually only one or two touring models, if that. So this will take a little research, but I think will be worth it in the long run. A good tourer will take you through your commute on the weekdays, into fitness rides on the weekends, and even into beginning club rides.

Do some reading, research, and educate yourself so you can shop wisely. Make sure you go to a shop that will spend some time fitting you properly.

And definitely post back and let us know what you get and how you're doing. Welcome aboard!