I think it's a great challenge. It's just painful and just doable enough to make a good challenge. We're not there yet, since we're a family with small kids and have commitments to keep that require a car. However, I've thought of trying to go car-free for a month.
By the way, if you do the kind of driving it sounds like over the next month, you'll probably not be driving at your most efficient, so you likely have less than 20 miles. That's just a technicality, though, and I think your 20 mile limit is a great one.