London Marathon event director Hugh Brasher is expecting some "incredibly quick" times in Sunday's 40th edition of the event.
This year's races are restricted to elite athletes only with the usual 45,000 runners unable to take part due to the coronavirus pandemic.
"Whatever the conditions, we believe there will be some incredible racing that will live long in people's memories," said Brasher. "It could be incredibly quick."
Mo Farah announced last month that he will act as pacemaker in the men's race, in which Kenya's four-time winner and world record holder Eliud Kipchoge goes head-to-head with Ethiopia's Kenenisa Bekele.
Brigid Kosgei, who broke Paula Radcliffe's long-standing world record in Chicago in October, will headline the women's field.
Course record holder Manuela Schar is chasing her 10th straight win at a major marathon win in the women's wheelchair race.
David Weir heads British hopes in the men's wheelchair race, in which American Daniel Romanchuk is a hot favourite to defend his title.
"We have some athletes in great shape, but they've all been training in unique conditions," Brasher said.
"So it is probably the most difficult marathon to predict. This is the only major marathon event to take place this fall, there are so few events happening.
"In athletics at the moment we've seen some amazing performances this year from athletes who were going to do the Olympics and haven't and were really training hard."
Non-elite runners with a place in the 2020 race – no longer able to participate due to coronavirus – will compete in virtual marathons in 109 countries and can monitor their progress on the event app.
Brasher said there had been 456,000 applications in only six days for "the most popular marathon on the planet".
"We are delighted to be able to hold this event with the world's greatest athletes and at the same time with 45,000 people in the largest-ever virtual marathon in the world," Brasher said.
"It will be a 40th race that we believe will live long in the memories for being different, but for being appropriate for the world that we live in.
"Even though those 45,000 are doing the race apart, they are still doing it as one community and that community is being built through social media, through the app that gets released later (on Tuesday)."
Brasher said the 2020 London Marathon was fully supportive of the Black Lives Matter movement, but a decision over whether runners would be taking a knee would be an individual choice.
"Each athlete needs to determine what they think is right," he added. "Encouraging people to do what they think is right, and to do it with integrity, is to our minds really important."
