http://www.sueandpaul.com/gmapPedometer/
Check this site out. It's a tool for measuring running routes that taps into Google Maps. Once you get the hang of it, it is surprisingly accurate. I checked it against a 9.3 mile route that I ran this morning (which I have measured more than once in two different cars), and it calculated the distance to be 9.25 miles -- really close. I live in an area with curvy roads, and the tool measures by point-to-point as you instruct it... so it's a little hard to trace curves by straight lines (the trick is to make the lines between points really short on curvy stretches).
You can also use TinyURL to make convenient bookmarks for your routes... My route from this morning is here as an example:
http://tinyurl.com/8sx2m