I found this great website on Martins Money Tips website.

It provides you with a walking route in London.  You just type in where you want to start and where you want to finish and it tells you how far it will take depending on whether you are a slow, medium or fast walker plus the calories burnt.
http://www.walkit.com/

Another site is www.walkjogrun.net.  It starts off by showing you a map of America but you just type in your postcode or town and it will take you to Britain.
Again it is great for telling you the distane, pace and number of calories burnt