Just a quick post to show you the route that I took home. This is the map that I had with me (thanks to the wonders of the iPhone and Google Maps!), and apart from one or two last minute changes (read: "going the wrong way...") is pretty much spot on with how I got home. As you can see, it's not by any means the most direct route! But it does avoid having to run down the M1...
End of the marathon in St. James' Park to join the Greenwich Meridian Trail.
Basically follows the A10, gradually becoming less and less busy until I made it out of central London. Met up with Dave and Pete at Mile End, Forest Rise, and another place that I can't remember...
The Greenwich Meridian Trail through London, up over the M25 to Waltham Abbey
Waltham Abbey to Stansted Abbets
Stansted Abbets to Standon
Standon to Wyddial
Wyddial to Royston
Royston to Orwell
Orwell to Hardwick
Hardwick to the Guided Busway (via Bar Hill)
Home straight - Guided Busway into St. Ives