I feel the need to add a few things.
I've read repeatedly about this superspreader runner. Not sure were you got that from. He is "famous" because he was the first local citizen diagnosed with covid-19 (we had chinese tourists already in the hospiltals by that time) - not so much because he brought down entire towns by himself.
Another myth is he was super fit. Think 1:50 HM to put this in perspective (I'd call it above average - but not more).
As this whole thing moves on we are gathering a lot of data and statistics start to be meaningful. In regard to the way of transmission some interesting studies are going on. Many things point to the fact that inhaling the virus is the way most people catch the virus.
This is backed up by numbers.
Bergamo had a big spike in positives some 10 days after their team played their game in the UEFA CL.
Tourists from all over Europe are now suing alpine skiing resorts for allowing infected barkeepers continue to work in crammed apres ski bars and clubs. Many cases from Iceland, Denmark, Netherlands, Germany and Austria can be traced back to some tourists partying in Ischgl.
Carnival gatherings in Germany are also seen in a different light in the aftermath now. Seems to be the same with Mardi Gras over in the States.
In Germany prof. Streeck is doing an interesting research on these things. Apparently it is very hard to catch the virus by touching surfaces. They went into houses with infected people and could only find dead RNA on surfaces.
So back to the topic - I have my place in the 2020 London Marathon. A lot of things need to happen for that kind of race to take place. Think about the 30 minutes before the start. 40000 people crammed together. Closer than any rock concert I have ever been to. To consider also the breathing/snorting while running as rojo said.
A mass event in running really has one of the lowest priorities and one of the highest risks at the same time.
What about elite only:
I can see London pulling this off for one year. In the longer term you have to consider the business side. Elites is where they spend the money (marketing budget so to say). Mass participation is where they make the money.
On a sidenote:
I've read various statistics about numbers of deaths in march '20 compared to march'19. There are huge outliers. If you add in that the shutdown almost eliminated all the deaths from traffic and leisure activities for most of the month this does not look good.
Regards