Must be the weekend for 20 mile runs. I did one today and I feel like I've been drained of life. That heat just zapped it right out me. I should have just quit at the top of the big hill I ran up which was full of cars stopped on it watching boats sail down the Clyde.
Fazza - I agree with Emmy. London is very crowded, so if you are an average paced runner (I was mostly running at just over the 4hr mark) it can be very difficult to find the space to just go at your won pace. Particularly in the opening 6 miles. I was walking after just 200 metres for a minute in London as the crowd just came to a sudden halt. People walking on narrow roads right from the start area blocking folk behind them.
To be honest I didn't really notice the turns in London. Although at one point I did think Canary Wharf would never end!
I've ran both and prefer Paris too. The course is more interesting in my opinion and it's far far easier to run at your own pace in Paris. It doesn't have the constant crowd support you get in London though.