Thousands of runners are due to take part in this year's London Marathon, now in its 36th year, and joining them via a treadmill in the International Space Station will be British astronaut Tim Peake.
Participants will pound the 26.2-mile course round the streets of the capital today, while Major Peake will be able to watch the roads pass under his feet in real time on an iPad.
The 44-year-old, who ran the Marathon in three hours 18 minutes and 50 seconds back in 1999, has recorded a good-luck message for his fellow runners.
The message will be shown on a big screen at the event, and ends with a 10-second countdown.
A host of celebrities are signed up to run for various good causes, including Game of Thrones star Natalie Dormer, who is running for the NSPCC.
Others include Chris Evans, who is taking on the challenge for the second year in a row, raising money for Children in Need, and Dame Kelly Holmes.
The double Olympic champion is making her London Marathon debut and aims to raise a quarter of a million pounds for a number of charities, including one she founded to help disadvantaged young people.
A veteran of the race is Iva Barr, 88, from Bedford, who is running this year's Virgin Money London Marathon for disabled young people's charity Whizz-Kidz.
Rebecca Manners from Manningtree in Essex will celebrate her 18th birthday by running the route alongside her parents, in aid of VICTA, which supports blind and visually impaired children.
Runners face a chilly start with a high of just 12C (54F) expected, but the snow predicted by some in recent days is highly unlikely, according to the Met Office.
This year's Marathon can count a milestone - its one millionth competitor will cross the finish line in this year's event.
In the elite men's race, Kenyan Eliud Kipchoge will aim to defend his 2015 title after he pipped former champ Wilson Kipsang to the finish.
Meanwhile, in the elite women's race, Tigist Tufa will be hoping for a repeat performance of her thrilling finish last year. The Ethiopian outran the favourites to take the 2015 title.
Prince Harry, patron of the London Marathon Charitable Trust, will present the winners' prizes.