Given the length of your ride, I might consider a road bike with drop bars. It will be lighter (20 lbs?). You can upgrade wheels which seems to be the one upgrade that people say makes the biggest difference. You won't be riding sitting up, but more bent over and you'll definitely be more aerodynamic. Since you already have a bike, you can take your time and find the right bike on craigslist.

Do you plan on adding fenders or a rack? Not all road bikes easily accommodate those.