From my ltd experience, you will certainly refine your commuting route and riding style leading to dropping in commute times. Towards the end of my commuting season (yes, I'm a weeny that doesn't like cold) I dropped about 15mins off a 10 mile one-way commute compared to when I first started commuting. Due to better shape, experience, route changes etc....
Don't let the first few rides deter you. As some have said, have a few trial rides. That way you will avoid any "gotcha" moments. I know, it happened to me.