It occurs somewhere around 17 miles. By then, there are still nearly 10 miles to go, muscle aches become a serious pain, it's too hot to trot and runners are entering the unlovely Isle of Dogs. But they meet a reception like a team who have won the Cup final.
"About 10 years ago, you only got a few people going along. Now this is a very big event in the area," says Louis St Clair, whose disco blasts out from Drummonds on the Quay pub. For East Enders, the marathon has become an excuse for a street party.
Along the street for hundreds of yards, crowds gather three or four deep to encourage the runners - the older, slower or sillier, the better. Thousands of balloons help the party along but it's the locals, who cheer and clap thousands of people they've never met, who turn the Isle of Dogs into Partyland.
Jill Butler, who ran the race in 4hr 37min last year, says: "I loved it when I got here. It's so nice to be cheered and hear your name called out. It really gives you a lift." Jill, who just beat Mr Blobby, is among dozens of runners from as far away as Falmouth and Fife who swell the Docklands Supporters Club. Geoff Dennis of Reading says: "This is where you start to feel it. You can see your runners pick up with all this support."
Betsy Eisenhower, from Chiswick, west London, hands out slices of oranges and sweets to runners. She's paid for them all herself. "I've come to watch my husband run, but I ran the race last year and it's such a great atmosphere here. I've saved the sweets and oranges for these people (she points to the five-hour brigade dawdling through) because they need it most."
By 17 miles, the runners have sorted into peer groups. After the wheelchairs, the top men and women, the club runners filter through: first a trickle, then a swarm and finally a mass that fills the road. The first are running easily, the next sweating a bit, then come the ones whose faces say: "This is not fun after all."
As the procession unwinds, the grimaces get worse. Soon everyone carries a water bottle. It makes an odd sight. An alien landing at Marsh Wall yesterday would think that a huge crowd of people in underwear were fleeing an unseen monster, and that humans drink water through the tops of their heads.
Now's the time the locals have been waiting for. First the silly hats, then the daft T-shirts, and eventually the rhinos, clowns, cowboys, chefs, cartoon characters and full-scale crackpots, wearing steamrollers, brides on their backs, linked as caterpillars or carrying ladders.
They are flanked by the also-rans, though that's a bit of a misnomer as most are walking and look likely to do so all the way to the finish - if they get that far.
But even these tired souls, who wear a watch to tell the time rather than to pace their race, raise a gentle jog at the Docklands reception.
These stragglers are the ones who will use most of the 88lb of petroleum jelly, the 1,300 plasters and the 1,000 vomit bags that are part of the pre-race checklist. But afterwards they will be able to boast: "I ran the London Marathon (or at least a few yards of it in Docklands)."Reuse content