Well, couldn't find anything on the 2-mile rule -- I know I'm not making it up, I saw it on some website, somewhere. But I'm going to post on it, anyway, to get it out there since I'm already using it in my posts.
The 2-mile rule for cycling is, simply put, using your bicycle for trips that fall within a 2-mile radius from your house. On this now-mythical website the person (forever unknown) gave a simple way to calculate this using a map and a compass. Using your bike computer (if you don't have one they can run really cheap to really expensive) would be a definitive way to figure out distances. Mapquest and Bikely.com would also work well for figuring out distances to some of your more frequent destinations.