After months of gruelling training runs, 35,000 runners are counting down the hours to reaching the start line at Sunday's Virgin London Marathon.
The event may be just the second most important 26.2-mile race in the capital this year, but for many of the entrants it will be their ultimate achievement.
The 32-year-old marathon provides a rare opportunity for fun runners and celebrities who could only dream of entering the Olympic Games to compete on the world stage.
Meanwhile the elite athletes - who will have their eyes on this year's main prize - know success in the London Marathon could persuade national selectors to enter them for the 2012 Games.
After Paula Radcliffe, Scott Overall, and Mara Yamauchi were named in Team GB's marathon squad in December, there are just two slots available for the men and one for the women.
Overall will run as a pacemaker for the leading British men aiming for the Olympic A qualifying time of two hours and 12 minutes.
"It was fantastic to beat the qualifying time in Berlin and to be selected for the Olympic Games in my home city," said Overall, who was a surprise fifth in Berlin in a race won by Kenya's Patrick Makau in a new world record.
"But I don't want to be the only British man on the start line on August 12. I want to have friends and team-mates beside me and am happy to do whatever I can to help them beat the time.
"Of course, it will also be a good opportunity for me to have a serious workout on the London Marathon course as part of my preparations for the Games."
Lee Merrien, who was the first Briton across the line in last year's event, is one of the main contenders.
Claire Hallissey and Louise Damen are among those battling for a place on the women's team. They have both already achieved the qualifying time of 2hrs 31m.
In the men's race, Emmanuel Mutai will defend his title against five fellow Kenyans all vying for places on their country's Olympic team.
Mutai smashed the course record to win in 2:04.40 last year, but will need to be at his best in a field which includes 11 men who have run under 2:06.
In the women's race, Mary Keitany will defend her title as she and her fellow Kenyans also look to secure Olympic selection, with the likes of world champion Edna Kiplagat, silver medallist Priscah Jeptoo and Berlin winner Florence Kiplagat all in the high-class field.
Britain's David Weir will defend his men's wheelchair title as he seeks to match the six London Marathons won by Baroness Grey-Thompson on his 12th consecutive appearance.
"It's gives me a real boost to know I can equal Tanni's record this year," he said.
"Tanni is such a fantastic ambassador for the sport so to match one of her achievements will be fabulous.
"I love the London Marathon. I haven't missed one for 12 years and I wouldn't miss this one for the world."
American Amanda McGrory will be hoping to repeat her success of last year after taking nearly two minutes off the course record in the women's wheelchair race.
The London Marathon always attracts a number of famous faces on the start line, and shadow chancellor Ed Balls has said he is just hoping to get round the course.
Mr Balls said his wife - the shadow home secretary Yvette Cooper - thinks he is "completely mad" to attempt the run.
The Morley and Outwood MP is running the marathon for the charities Action for Stammering Children and Whizz Kids, which provides equipment to help disabled youngsters live full lives.
Forecasters are predicting a dry and bright start to Sunday's race, but warned there was a chance of rain later in the day.
Temperatures will be about 8C (46.4F) as the runners set off, reaching 13C (55.4F) later.