Can Kenenisa Bekele dethrone marathon king Eliud Kipchoge in the 2020 London Marathon elite-only race? And can Brigid Kosgei hold off world champ Ruth Chepngetich and defend her title? Here's how you can watch live on Olympic Channel
Kipchoge is the defending Olympic men's marathon champion, world record holder, and the first person to run a marathon in under two hours.
"When you work hard and when you believe in yourself, anything is possible." - Eliud Kipchoge
But Ethiopia's Kenenisa Bekele is a three-time Olympic gold medallist, five-time world champion on the track, and the reigning 10,000m world record holder.
Just two weeks before Kipchoge's historic sub two-hour marathon in Vienna, Bekele won the Berlin Marathon in 2:01:41, two seconds outside his rival's world record set at the same race 12 months previously.
Now they will go head-to-head in a special London Marathon race for elite athletes only due to Covid-19 restrictions, with Bekele's former track rival and four-time Olympic champion Mo Farah among the pacemakers helping runners beat the Olympic qualifying mark of 2:11:30.
Like Kipchoge, world record holder Brigid Kosgei is back to defend her title in the British capital.
Reigning marathon world champion Ruth Chepngetich and Olympic champion 5,000m ruuner Vivian Cheruiyot are two of the leading contenders in the women's elite race.
The men's and women's runs are on Sunday (4th October), as well as elite wheelchair races, take placed on a closed loop rather than the traditional street circuit.
Find more on that, the key rivalries, and how to watch the race below.
2020 Virgin Money London Marathon
Eliud Kipchoge is already a four-time winner of the London Marathon.
Before leaving for London on Sunday, he told NTV, "I am really feeling good having trained for the last three months and now I am waiting for the big day. I have already taken a Covid-19 test where I have been cleared having tested negative. I am eager to go and represent my country in the London Marathon."
And in a video, he said, "Above all I need to show people that even in times of uncertainty, you need to aim high, to focus and achieve their goals.
For years, Kipchoge and Bekele battled on the track - with Kenenisa usually winning the critical races.
The Kenyan did win the world 5000m title in 2003 as an 18-year-old before Hicham El Guerrouj beat them both in the Olympic final at Athens 2004.
Four years later, it was Bekele - Ethiopia's successor to Haile Gebrselassie - who sprinted clear of Kipchoge in Beijing to complete a long-distance double in the Chinese capital.
Kipchoge soon moved up to the marathon as Bekele stuck with the track where he soon found himself behind Mo Farah in major global competitions.
The Ethiopian hero eventually made his switch to the road, but their clash at the 2016 London Marathon failed to produce the expected fireworks as Kipchoge won comfortably with Bekele back in third some three minutes adrift.
Bekele has worked hard to raise his level, and his performance in Berlin gave notice that he could be a genuine threat to Kipchoge's unbeaten run of 10 marathons.
But there is little doubt Kipchoge remains the overwhelming favourite.
In April 2019, he produced an awesome front-running display to shake off the attentions of three Ethiopians and win his fourth London Marathon.
There could be an element of team tactics this time round with Mosinet Geremew and Mule Wasihun, who chased Kipchoge home last year, and Shura Kitata among the Ethiopians in the field who have run inside 2:05.
Both protagonists spoke to the Olympic Channel Podcast in April ahead of their latest mouth-watering contest over 17 years since their first meeting.
And Bekele, now 38, says he believes Kipchoge can be beaten.
"Of course. He is human. If someone prepares well and is strong enough, why not? Eliud Kipchoge is human like all of us. So why not?" - Kenenisa Bekele on the Olympic Channel Podcast
Brigid Kosgei heads a strong women's field dominated by East African runners.
The defending champion spearheads a formidable Kenyan team which includes 2018 winner and Rio 2016 5000m gold medallist Vivian Cheruiyot, reigning world champion Ruth Chepngetich and last year's Frankfurt victor Valary Jemeli.
Leading the Ethiopian challenge are last December's Valencia Marathon winner Roza Dereje and 2019 Amsterdam Marathon champion Degitu Azimeraw.
Kosgei showed she was in excellent shape at the Brussels Diamond League three weeks ago at the attempt to break the one hour world record.
But, in her first official track race, Kosgei was disqualified for stepping inside the kerb with second place going to Kenyan-born Israeli Lonah Salpeter instead.
There are also two wheelchair elite races in London with Manuela Schar seeking to pick up where she left off in 2019.
The Swiss won nine consecutive World Marathon Majors but coronavirus considerations forced her to miss March's Tokyo Marathon.
She told Abbott World Marathon Majors in April, "We have months without any race. It's a totally new situation. I usually need that motivation for my training, that competition feeling. Especially with the Paralympics postponed to next year."
But her training is clearly going well as, in August, she broke the five-year-old 1500m T54 world record on the track in Nottwil, Switzerland.
Schar's main rival could be two-time London Marathon winner Shelly Woods who makes her first appearance in the race for four years following the birth of her son.
With reigning champion Daniel Romanchuk a late withdrawal, the men's race could come down to a duel between twice winner Marcel Hug and home favourite David Weir.
Now 41, Weir will be taking part in his 21st London Marathon and going for his ninth victory having last been victorious in 2018.
His old Swiss rival Hug, aka The Silver Bullet, is the reigning Paralympic marathon champion having also won the 800m on the track in Rio.
As well as being staged six months after its usual April date, the 40th London Marathon will not be held on its normal course around the streets of the city alongside the River Thames.
Instead, it will take place on a 2.15km closed-loop course in St James's Park, in the centre of the capital, starting and finishing on The Mall.
The field will cover 19 laps before a final three-fifths of a circuit to complete the marathon distance of 42.195km (26 miles and 385 yards).
In order to maintain a biosecure environment, there will be no spectators lining the course, with fans encouraged to watch television coverage.
The whole race is available live on Olympic Channel (territorial restrictions apply). Watch here on Sunday.
Non-elite runners will be able to compete in a virtual mass event with over 45,000 signing up from around the world.
07:15 Women's Elite Race
10:15 Men's Elite Race
13:15 Wheelchair Race
Each race will have no more than 40 participants to ensure safe social distancing.
There is a virtual mass participation event with runners who had been accepted for the 2020 race deferring their 'real' entry to one of the subsequent three London Marathons.
You can watch the action from the 40th London Marathon in selected territories live on Olympic Channel.
2020 Virgin Money London Marathon
Brasher's participation in the 1979 New York City Marathon was his inspiration as he wrote in the Observer, "To believe this story, you must believe that the human race can be one joyous family, working together, laughing together, achieving the impossible."
His dream was realised two years later with 7,747 making the start line from over 20,000 applicants.
The finish befitted the occasion with American Dick Beardsley and Norwegian Inge Simonsen crossing the line together in a deliberate dead heat.
Wheelchair events were added in 1983 with the London Marathon becoming one of the most popular marathons in the world with over 40,000 runners taking part each year.
Its rise also hastened the demise of the Polytechnic Marathon, first run in 1909, which standardised the distance for the race.
London has seen its fair share of world records over the years courtesy of the late Grete Waitz, Ingrid Kristiansen and Khalid Khannouchi who beat track stars Haile Gebrselassie and Paul Tergat in 2002.
A year later, Paula Radcliffe smashed her own world best with a time of 2:15:25 which stood until Brigid Kosgei broke it in Chicago last October.
Kristiansen is the only four-time women's elite race winner with Kipchoge the sole four-time men's elite winner.
David Weir's eight victories is a record for a wheelchair athlete with another Briton, Tanni Grey-Thompson, the top woman with six wins.