According to the route and elevation profile from my 620, last year's london marathon involved running up to the top of Canary Wharf tower, diving from the top into the docks and swimming across before rejoining the course. Don't rely on autolap for pacing, use the mile markers as Emmy says.
I think they thought about canning it TR, but it was on a closed circuit with plenty of room for everyone. At least two got blown over off their bikes though. I wouldn't have fancied racing on a bike on open roads today.