Be prepared for congestion at the start and maybe work that into your training, with starting off slower and speeding up for the second half.
Not sure why they get so much congestion at London . I did Paris in 2009, similar numbers and only 1 start area. I was in the 4 hr pen ( not right at the front) and hit mile 1 at exactly 9 mins. There was also a lot less people walking after 5 miles also. Maybe some of us Brits think we are better than we actually are.
Edited: 08/01/2013 at 13:34