I've done 2 Londons (95, 97).
Just some comments I'd like to add to the others.
i) You have quite a twisting/winding loop through Jamaica Wharf at about 7
to 8 miles...find the centre of the road and stay on it if you can,
otherwise the changing camber as you cross over at corners etc can make
itself felt on your knees later in the race.
ii) Watch out for the spilt Isostar..they hand out 500ml packs and people
generally only drink a couple of mouthfuls...so what happens to the rest....
you guessed..it gets dumped on the floor..I've seen several people slip and
one fall on this stuff..hell of a way to finish your marathon with a twisted
ankle at a feed station :-(
iii) The trek out to Docklands/Canary Wharf can be very depressing!!!! As
well as very WINDY. In London in April the weather can also be COLD (i.e.
snow/sleet/rain), it can be windy,especially by the river...any wind can
just blow you away in terms of time...its very sapping.
iv) You've been told about the cobbles. The jog along the Embankment can
either be a joy or a pain..its long, straight and UNDULATES..not much but at
23 miles in a marathon it can be like Mt Everest. Time to dig deep and tough
it out...the crowds here are usually great!!!
v) The final PULL up the the houses of Parliment where you turn right...as
above , dig deep!
vi) Nicest moment..turning the corner to the finish...NOTHING feels as good
Its a great event. Enjoy!!