Originally Posted by
NormanF
If you want to race fast, get a real road bike.
Country/Adventure/Gravel Grinder road bikes are not built for speed. They're built for comfortable all-day riding, to let you haul stuff and to get you anywhere you want to go.
So if you want to race someday, spend the money to get a light and fast bike just for the road.
Don't try to convert another bike because you won't have as much as fun riding it and it will never go fast.
Not quite true. I have no trouble matching my synapse (endurance bike) against a super six evo (race geometry) on spirited group rides. Also, I think if you are more comfortable, you won't get as fatigued.
OP: I would get yourself a road bike. Skip the racks and stuff. Commute on your heavier mtb, but take extra clothes and stuff with you at least once a week and enjoy your commute on your "fast" bike. The extra weight of riding your commuter will make you stronger. I have a similar distance commute and ride it on a very heavy bike most of the time, and like to bring extra clothes and ride in on either my 89 Schwinn or my carbon Synapse. It's just fun to do that sometimes.