A race around London

[BigAl]

Here's a fun challenge that will take from 10 to 30 minutes to complete.

Load up Google Earth (version 5.xx or above) and go to the Dartford Tunnel at the south exit of the M25 motorway.
Zoom in close enough that you can see vehicles on the road.
Now begin scrolling the view so that the M25 remains on the screen.
Continue clockwise around the entire M25 till you come back to the Thames on the north exit of the Dartford Tunnel.
How long did it take?