My semi-usual weekend walk in Fairmount park is about 4 miles (2 miles in each direction) plus a little extra getting there from where K. does yoga. It's a good workout for me, especially on a hot day, but it doesn't wipe me out or anything.
Now, if I stopped taking the subway at end of the day, that walk (from West Philly to South Philly) would be 3.7 miles, which I did both ways for about a week during the transit strike in November 2005 and it just took me an hour (maybe even less). And that's only 35 minutes longer than it takes me (door-to-door) when I take the subway.
If I did that four days a week, it would be a big boom to my new fitness regime, plus it would give me quality time with my ipod and lots of time to think about my writing. The only downside would be eating dinner kind of late and getting less reading done.
I think I'll give it a try.