A record number of runners are primed to tackle this year's London Marathon, where a trio of royal cheerleaders will spur them on from the sidelines.
The Duke and Duchess of Cambridge and Prince Harry will be dotted along the 26.2-mile course to lend support to the 40,000 competitors pounding the streets of the capital.
Earlier, the royals, who champion the official charity of 2017's London Marathon, Heads Together, will push the red button to start the elite men's race and mass race.
Hundreds of police will be on duty to keep around 800,000 spectators and runners safe, barely a month after crowds were targeted in the Westminster terror attack.
Armed officers will also be sent watch over the crowds, but the Metropolitan Police said: "There is no specific intelligence that the Marathon will be targeted."
Organisers said a total of 40,382 people collected their race packs on Saturday, meaning the 37th event is poised to outstrip last year's record of 39,140 competitors.
Among the runners undertaking the daring feat this year is 47-year-old Gary McKee, who will be finishing his 100th marathon distance in 100 days.
With an election race in full flow, the annual challenge will also fittingly play host to a record number of MPs.
A total of 16 members - including five from Labour, nine Conservatives, one independent and one from the SNP - will join the throngs of racers.
A host of famous faces will be spotted undertaking the gruelling endeavour for good causes, including pop band Scouting for Girls, who are running for the Alzheimer's Society, and radio host Chris Evans, who is running for the third year in a row, raising cash for Children in Need.
Eastenders star Adam Woodyatt faces a more emotional challenge as he runs alongside his 19-year-old son Sam, just months after the teenager was hit by a car.
Olympic and Paralympic heroes will also make the journey from Blackheath or Greenwich to Buckingham Palace, including Rio gold medallist rowers Heather Stanning and Helen Glover, cyclist Chris Newton and Paralympic rower Pamela Relph.
Sixty-five years separate the oldest and youngest competitors in the Virgin Money London Marathon 2017.
At 83, Kenneth Jones will be the oldest runner, but is no stranger to the challenge - having competed in every London Marathon since it began in 1981.
He is part of a band of so-called Ever Presents who have done all of the London races, with 11 others joining him on the starting line again on Sunday.
Around 18 years after Mr Jones first attempted the event, this year's youngest competitor was born - Bronte Randle-Bissell, who will be running just three days after turning 18.
Runners face a bright, chilly start to the race, but will cross the finish line in warm sunshine and temperatures in the mid-teens, the Met Office said.
Tussling for the men's elite crown this year will be the second-fastest marathon runner of all time, Ethiopian Kenenisa Bekele and last year's runner-up, Kenyan Stanley Biwott.
In the women's elite race, Kenyan champion Mary Keitany vies for her third London Marathon title, coming up against fierce competition from world champion Mare Dibaba and Britain's Jo Pavey, who is still competing at the age of 43.
British six-time wheelchair winner David Weir has a record seventh London crown in his sights amid speculation about retirement.
World records will also be attempted by around 50 runners, including the fastest marathon in a sleeping bag and fastest marathon in Wellington boots.
William, Kate and Harry said they hope it will be the year of the "mental health marathon", with blue headbands likely to be worn by many competitors this year in support of Heads Together.
Some fundraisers will receive medals from the royals as they cross the finish line on The Mall, while Harry is to present trophies to the top three elites and World Para Athletics Marathon World Cup competitors.
The races start at Blackheath and Greenwich Park from 8.55am and end in the shadow of Buckingham Palace.
Road closures will be in force and there will be delays to some public transport.