I've been a year-round commuter for several years; a new job assignment in March changed the ride from 4-5 miles (~8km) 1-way (+ whatever longer routes I added), much on a bike trail/MUP to 11-12 miles (~20 km) 1-way, entirely on roads. So I didn't have to ride into shape, I did have to get used to 3x the daily mileage. I had been taking an indoor cycling class (Computrainers + Erg videos) and used the riding to extend some of what I learned in the classes.

Lots of good advice above. The day after day riding will help your endurance (and burn off a lot of excess pounds as long as you don't go crazy eating), although I'd still work up to even longer rides if you also plan to ride, say, a metric or English century. You can work in intervals on some days to build speed; use favorable sections of road or stop lights - they won't be organized structured intervals like the training books describe, but they will fit easily into your commutes. Either way, you'll feel like you are flying if you go for a ride without the commute load.

Definitely work those hills into your routine. It will help both fitness and ride time, even if you take it easy the rest of the way. Consider using a faster cadence - I've trained myself to spin in a lower gear - I'm usually doing 90-100 rpm (or more) unless it's really steep. It's easier on my legs, if nothing else. YMMV - I know people who ride quite well at lower cadences - my "motor" may simply work better at a higher RPM than their's.

If you like numbers, keep a log of your commutes. I've noticed over the past 5 months several definite trends: Average speed has increased, average heart rate has declined (including plotted against speed), and I've dropped a few more pounds. Good luck!