It always measures a bit long on GPS watches. My readings have been 26.8, 26.76, 26.66 and 26.62, and they make up 4 of my 5 longest marathons. There are varying reasons for all of them, ranging from toilets which are about 100m from the course (or underground in Iidabashi station!), the GPS regularly going a bit mental in Shiba, Ginza, Kyobashi, Nihonbashi, Asakusa and Tsukiji (so thats quite a lot of the course!), and taking what I like to call "the anti-racing line" around some of the many corners and hairpins.
Ive done this race 4 times. The start area looks more confusing than it actually is. The vast majority of the event village is open-air and the only "indoors" area is an underpass beneath the metropolitan building. Since the start line is virtually above the event village, you'll need to go upstairs to the blocks, but everything is outside.
Everything is very clearly signposted in the event village in Japanese and English, and the announcements over the PA system are loud and clear, again in both Japanese and English. Even though the diagrams in the handbook are quite confusing, it'll be pretty straight-forward once you get there.
Also, the marshals/volunteers for this event are all brilliant. Happy, friendly and helpful. Talk to one if you dont know where to go.
Once you get to your block, you're probably in for a 30 minute wait while they go through the formalities, and then you're off.
Having done Hever last September, I can safely say Ive never experienced water as cold as that lake at Hever. Even if it was above 15 degrees - and I wouldnt be so sure that it was - Id wear one purely for the added "warmth" (if you can call it that!). I dont think I can sufficiently emphasise just how cold that water was! Even the thought of jumping in there without a wetsuit makes my goolies attempt retraction!
The organisers of Tokyo marathon like to make it tricky to find the handbook and hide it away in some dark, not-very-easily-accessible corner of the website. They do this every year. Here it is for those who are fed up with endlessly navigating the site: