Thanks to Tom, I’ve been alerted to gmap-pedometer.com. It lets you manually enter routes (say) for running and calculates your mileage to a degree that is way more accurate than you need. It gets you down to the 1/5th of a foot, which is a bit silly, in my opinion.
This is neat, although time consuming if you don’t live in a place with absolutely straight routes. You need to click on each curve point, or Google will draw straight lines across the intervening territory. While this would be an entertaining run, I don’t advocate it.