This is easy to determine on the trails in Forest Park with all the cool maps available but a bit of a pain when you are doing road runs.
In the old days I would run my route and then hop in the car (usually after a shower) and drive the course to determine the mileage.
This had numerous problems:
- any parts of the route that were through parks or on trails you had to guess
- more time away from the family
- it did not help in explaining to friends the route you just ran
- it was boring
I know of two: USA Track and Field and one associated with the Eugene Marathon (which I ran last year).
Both are powered by Google so you get great street and satellite views.
The Eugene one (really RealBuzz.com) requires you to create a user and log in which is mildly annoying. It has the additional feature of showing a Terrain view and handling GPS upload and export (I did not try the GPS stuff). I could find no way to show elevation.
The USATF requires no login and no terrain view but does have elevation profiles (although today I could not get any elevation profiles :-( ).
These are great as they provide you with mileage, possibly elevation and you can save the mapping and send it to a friend to brag about your great workout.
A little way to make the training easier :-)