Neighborhood running junkies and mile-tracking-aholics rejoice, because Gmaps Pedometer has got your fix. Click your route*, block by block, and the mileage is posted on the screen. The plan I am roughly following is based on mileage, so I have found the tool to be indispensable in keeping me accurately on schedule.
To begin you enter your starting location, zoom in, then hit the "start recording" button to start logging your route. Click once and drag to move the map and double-click to mark a stopping point, turn or "leg." Final results can be printed, saved, or exported.
Note: There are lots of other "Google Map Pedometers" out there, but personally I have found this interface to be the most friendly to use.
*The route above maps my walk home from work, albeit an infrequent occurrence in reality, but look--3.68 miles!