This is a website that Kevin, "the Commish", showed me. I've been trying to work out to get more in shape for a weight loss contest at work.
Gmap Pedometer references any Google map and allows you to plot out jogging paths. This lets you get more accurate paths that you travel then putting in addresses into Google Maps. You simply double click and then double click on a second point to get the mileage. This site is great for my neighborhood where I often cut through the middle schools parking lot when I'm roller blading. You could never do that with Google Maps because it was not a navigable way, but with Gmap Pedometer, you just have to chart it out.
The site also offers a calorie counter depending on your weight and distance you traveled (I never find those accurate), plus an elevation readout between two points (don't find it too accurate or helpful, I can tell more from my huffing and puffing that I have an incline).
You can also save your paths for later reference (I paste mine in a spreadsheet with the date and time of the run). It is a great little tool to get an accurate measurement of adventures off the beaten path.