As90 - sounds familiar, I'm down towards Greenwich direction. My standard 20 mile route starts at Kew and goes down the river enduring at Greenwich. You do have to get a bit pointy-elbowed round about Westminster and the Tower of London, but apart from that it's a nice run.
Orbutt - more than one person missed the Eiffel Tower .
as90 - running the Thames footpaths? We may have met . There's a few of us who are London based - which bit are you in? Agree with others it's a good first marathon and well organised (with a few bizarre aspects to the organisation that makes it uniquely French) but stick around and we'll keep you right. All your questions can be answered here - fire away.
Kirsten - if you're considering Edinburgh, be aware that most of it's not actually in Edinburgh. They take you straight out of the city centre, then it's an out and back along the coast through East Lothian. It's not actually the most scenic of races.