Let me start off by saying that my full commute is 50 miles round trip, but on some days, mostly in the winter, I split it, driving 15 and riding 10. My previous commute was 90 miles round trip, so I did a 30/15 split most of the time.
First...don't get some cheap wallyworld bike or it will definitely end up collecting dust. You'd be better off buying a higher quality used bike in that price range. Most important, make sure the bike fits you.
Second, find a bike that you'll enjoy riding, not something that makes it feel like work. A comfort bike isn't going to be very comfortable for over 10 miles, and not very fast at any distance. You might want to consider a hybrid with flat bars and a rigid fork. You can find them brand new in the $400 range. Most will accept larger tires, racks and fenders, if you opt for them later based on your commuting experience. Trek's 7.2FX is one example. I'm sure others with chime in with others.
Once you start increasing your distance, you'll probably find that a dropped-bar road/xcross/touring bike is better suited, but the hybrid is a good place to start and can be retained when you upgrade as a bad weather bike.
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"Let us hope our weapons are never needed --but do not forget what the common people knew when they demanded the Bill of Rights: An armed citizenry is the first defense, the best defense, and the final defense against tyranny. If guns are outlawed, only the government will have guns. Only the police, the secret police, the military, the hired servants of our rulers. Only the government -- and a few outlaws. I intend to be among the outlaws" - Edward Abbey