This year's postponed London Marathon could be staged with elite athletes only, says race director Hugh Brasher.
Brasher said that was one of 10 options that are "changing all the time", which could also see a further delay.
The event was due to take place on Sunday but has been moved to 4 October because of the coronavirus pandemic.
Nearly 43,000 runners took part in 2019, while Eliud Kipchoge and Kenenisa Bekele are set to go head to head in the 2020 elite men's race.
"We hope we can welcome them back in October but at this stage we just don't know what is going to happen," said Brasher.
"We have to look at it in totality, with three quarters of a million spectators, the medical side, the charities.
"We are looking at probably 10 scenarios, and they are changing all the time."
The Tokyo Marathon went ahead last month with elite runners only on largely deserted streets.
The Berlin Marathon, along with London one of the sport's six 'majors', will not go ahead as planned on 27 September after Germany extended a ban on large-scale gatherings until 24 October.
Brasher said he is in touch with other major race directors and some "radical ideas" have been discussed, but he did not reveal further details of those suggestions.