All good advice so far. I don't know if I can really add much, but I'll try.
21 miles is doable, but it's going to take a LOT of time, and you're going to have to work up to it. I know people who do 40+ miles a day, but man, it's not easy on top of a regular work day.
I agree with staying off sidewalks if at all possible. They are an accident waiting to happen. Do some research, it will almost always be faster and safer on the road. If you have access to trails, make extensive use of those.
If you're going to work up to 20+ miles, you're going to want a bike that's kind of fast, but more than that, it has to be comfortable. Race bikes aren't very comfortable, and they generally aren't set up for utility, either.
If money wasn't an issue, I'd get a Surly CrossCheck or Straggler. Fast, comfy, utilitarian. Full rack and fender mounts.
Mid-range, you can get a Bikes Direct road bike for about half retail, but you have to assemble and adjust it yourself. To me, well worth the money and effort. The Phantom Outlaw is a great bike for the money. It also has full rack and fender mounts.
Low-end, you can hit Craigslist for an old road bik, like a Panasonic, Peugeot, Trek, Giant, Fuji, etc. As long as it's in good working order, you can ride it.
Carry a spare tube (or two) and a small pump. Learn how to maintain the bike, and you'll be set.