There will be doping cases before Tokyo 2020 that will "shock people", says the head of the Athletics Integrity Unit.
Brett Clothier said more than a third of the AIU's cases are "World Championship or Olympic medallists or major marathon winners".
He added that, without action, the sport may have faced "rampant, unchecked doping" by marathon runners.
It follows a study which showed a lack of out-of-competition testing for top-three finishers in some 2018 races.
Clothier told the Telegraph: "We did a statistical study that showed that, in 2018, of all the marathon races around the world outside of the six major marathons, something like 70% to 80% of the podium finishers didn't have any out-of-competition tests in the nine months leading up to the race.
"There was just no testing. Not only that, but the athletes weren't in a testing pool so they knew no-one was going to test them."
The AIU was set up by the world governing body in 2017 to combat drug-taking in athletics, succeeding World Athletics' in-house anti-doping department.
Six major marathons have increased their funding to the unit, which has enabled it to expand its road-running programme and increase resources directed at the marathon discipline.
"It needed to be fixed or else the industry was going to face its own moment, like cycling did in the 1990s, or like baseball did [in the 2000s]. Rampant, unchecked doping," said Clothier.
In January, two-time London Marathon winner Wilson Kipsang was charged for "whereabouts failures" and "tampering" by the AIU.
The 37-year-old Kenyan is now banned from competing until his hearing has taken place. Kipsang's management stressed he has not failed a drugs test and added: "No prohibited substance was found."
Clothier added: "Based on what has happened in the past, yes, for sure there will be cases before the Olympics that will shock people."