I was moseying down the road one day at about 19 mph, feeling good and some guy on a mountain bike went flying by me. It was pretty flat, not windy and he sailed on by and kept going. Mountain bike. I imagine if he were on a road bike, he'd have sailed by even faster but the fact remains, the bike is not the problem. he looked like he had some shop kit on, so he probably raced in his spare time but still, mountain bike.

Get a lot of miles in your legs before you start obsessing about speed (say, 500-1000 miles). It will come.