congrats! You will find that your times will be the same on either of your bikes--at least they will average out to about the same. My commute is Reston, VA to DC - 22 miles each way, and my times are about the same on the fixie as on my multi-speed road bike. You will learn very quickly that 'momentum is your friend' as you approach uphills, and you will feel more and more comfortalble spinning at high speeds on the downhills. I ride a 48x18, and can reach 30+ mph on the downhills. I make every effort to push myself into the saddle, as that limits the 'bouncing around' that happens at high rpms.

enjoy and welcome to the 'club'

train safe-