I'm in VA, and I've only been commuting for about 4 months, and my commute is half the length yours is. I ride an old GT Vantara (Chrome-Moly, 21-speed, no suspension) that I bought for $100. I also ride with my kids in a trailer or on a tandem on the weekends; most weekends that means I ride about 7 miles towing 60 pounds or so.

So far, I really like larger tires--I'm running 38mm Michelin City tires, which are maximum width that would fit. (I've got about 4 mm of total clearance between the tire sides and the chainstays.) They smooth out the ride a bit, and I feel like the bike maneuvers better with them. (OTOH, the tires the bike came with, for which these are the replacement, were cyclocross tires.)