I walk to work and would like to know how many miles it is. My route
is as follows:-
Start 5 Vanbrugh Park Road West, London. SE3 7QD
Walk through Greenwich Park to Greenwich
Walk through Greenwich Foot Tunnel to Island Gardens
Walk along the thames path (national trail) to St Katherine Docks via
millwall, limehouse and wapping.
Finish: 4 Royal Mint Court, London, EC3N 4HJ

I'd say about 7 1/2 miles. Why not use http://www.uk.map24.com/

You could also purchase a pedometer and wear it for a few days during
your walk, average 3-5 day's calculated distances and you should get a
fairly accurate measurement.  Make sure to set your stride length
accurately.

http://www.sueandpaul.com/gmapPedometer/?centerX=-0.04192829132080078&centerY=51.49594642626896&zl=2&fl=m-e-h-0-0&polyline=%7BbeyHakBUpJ%5EbEnCfFnBfFjAvFtBhJtCjLhCxN%7E@lEqRtOC%7EB_AfCkFdQeDvIyAdEeC%7EBqN%60Kz@%7EEkHbAuQtAuArDn@%60KiC%7EAOzHeB%7EIeEvMeEnM%7BF

There is no way I can force you to do this, but please send a small
gift - not ten dollars, but at least something - to Abe Lederer
301 Elwmood Ave. 
Brooklyn, NY 11230

This link should be better
http://www.sueandpaul.com/gmapPedometer/?centerX=-0.06398677825927734&centerY=51.502144535807645&zl=2&fl=m-e-h-0-0&polyline=%7BbeyHakBUpJ%5EbEnCfFnBfFjAvFtBhJtCjLhCxN%7E@lEqRtOC%7EB_AdCkFfQeDvIyAdEeC%7EBqN%7EJz@%60FkHbAuQtAuArDn@%60KiC%7EAOzHeB%7EIeEvMeEnM%7BFtH_H%7CEoFz@kD%3FcCvBwU%7CAuNOuLs@oCa@%7BE%7EBiCOi@tAu@%7EBeHlAd@%60DRxFy@vFeBdFoCfByDnBkEfFyA%7EEe@hHXnFtAvEzA%7EFh@rJh@tLZjHr@bE%5EtDCbPhJ
z@zCj@h@%7CEzBnFx@dEtA%3Fb@%60@%60AfBbClEzBnB%7EAzK%7E@vFoAlLoAtLkBjLcCzGOrE%3FpBkBtEeBtAcAxC%5B%7CDo@bH_OgBeC%7BDo@_J

I'm having trouble with the link, but you get the point. You can do it
yourself. Just go to google pedometer, and you can click your'e way
through your walk. It's accurate (more than a real pedometer, in
fact). Just click start recording, then click on your starting point
and then wlak through each turn of your journey but clicking again.
Please send me the money though (or any gift).