Ask.com has this cool feature where you can calculate walking directions and distance. It's pretty spot on too. I just did my route to work and was going to show it until I realized that, um, I shouldn't post my home and work addresses online. I did do that walk today, though, and it takes about an hour. The estimate was 57 minutes, which I could definitely do if I stepped it up a bit.
So, here's a hypothetical walk from Pittsburgh to New York:
You can get there in 185 hours if you want! Pretty cool, huh?