Check out this website: http://www.gmap-pedometer.com/
It allows you to re-trace your run/walk/bike.
Go to your location and zoom in as far as you can. You can also use the sattelite view if you are looking for a trail. Click "start recording" button and then double click along your route.
For example, from my house to lake calhoun around and back it is approximately ~4.68 miles.
Here is my map: http://www.gmap-pedometer.com/?r=347478
It even gives a generic calorie calculation for running.