The talk started in earnest here in 2011, when Kenya’s Geoffrey Mutai ran nearly a minute faster than the world-best clocking on a hilly, twisty course. The conversation resumed two years ago when countryman Dennis Kimetto became the first man to break 2 hours and 3 minutes in Berlin.

So when does marathoning’s version of the four-minute mile finally happen? When does someone go under two hours? “Don’t get your hopes up,’’ cautions Ross Tucker, a South African exercise physiology professor at the University of the Free State’s medical school. “It’s not happening in the next 10 years. No way. In fact, I’d be surprised if it happened in 20.’’

The planet’s fastest time last year was 2:04:00. Tokyo, this year’s first major, was won in 2:06:56. Dubai, the sport’s version of the Bonneville Salt Flats, was claimed in 2:04:24. “We haven’t seen 2:02,’’ says Meb Keflezighi, the former Boston and New York champion who’ll be competing in his third Olympic marathon this summer in Rio de Janeiro. “It’s been kind of quiet. It’s more 2:06 now that’s winning. We’ll see how London goes, but with all of the allegations going on now it’s hard to predict.’’

The allegations involve Kenya’s running dynasty, which had 18 athletes sitting out doping suspensions as of January, including three-time Boston victor Rita Jeptoo. The Kenyans have set the last three world records. Were any of them to break Kimetto’s mark now, much less go under two hours, the time would be looked at askance.

“Our sport is facing a very difficult moment for us even to think about it,’’ says Wesley Korir, who won here in 2012 and will take the line in Hopkinton for the race’s 120th edition on Monday morning. “I do not think this is a discussion that should be happening right now . . . If someone runs under two hours I think I will say, that’s not true, that’s not possible.’’

For a while, as the global record kept dropping, the barrier seemed likely to fall sooner rather than later. It took a decade, from 1988 to 1998, for the mark to be lowered from 2:06:50 to 2:06:05. Between 2011 and 2014, it dropped from 2:03:38 to 2:02:57, broken by three different men.

“We got into this habit of expecting the world record to fall and it became routine,’’ says Tucker, who writes for the Science of Sport website. “But why should a world record ever be routine? By definition, it represents the greatest performance by a human being ever over the distance and it’s unrealistic to expect it to go year after year.’’

Unlike the mile, which is run on a regulation-sized track everywhere in the world, the marathon has multiple variables over a far longer distance, from course topography to pacing to weather and wind. “You never know in marathoning,’’ says Ryan Hall, who set the American record (2:04:58) here five years ago. “You’re in 2:02 shape and you hit a flyer and all of a sudden you have a great day.’’

That’s what happened here when Mutai ran an astounding 2:03:02 on a course he hadn’t seen until he arrived after acquiring a last-minute visa. All of the variables meshed for him. “It was a gift from God,’’ he said. The temperature was cool. The tailwind was an uplifting 20 miles an hour. Hall briskly led the way into the Newton hills. “He helps a lot because he pushed all the time,’’ said Mutai. “He was like a pacemaker.’’

And once Mutai reached the Brookline flats he had countryman Moses Mosop pressing him all the way to the finish. While his time wasn’t recognized as a world record by the international federation because of the course’s difference in elevation from start to finish, it got the sport’s clockers and watchers chattering about a global mark with a 1 in front of it.

The progression has stalled since Kimetto’s run, although Kenya’s Eliud Kipchoge (2:04:00) might have broken the record last fall in Berlin if he hadn’t run most of the way with his insoles flapping outside of his shoes, blistering and bloodying his feet. “It’s too early to know what’s happening,’’ says Tucker. “Certainly 2015 was the slowest marathon year in many and 2016 will likely be the same. Is that coincidence? Maybe. It could just be that an amazing core group of guys like Geoffrey and Emmanuel Mutai, Wilson Kipsang and Patrick Makau have reached the end of their careers and the next generation is not quite as good. That happens. Look at sports teams, right? The Bulls post-Jordan. The Heat. The Cowboys, et cetera.’’

Most observers seem to agree about how the two-hour marathon would happen if and when it does happen. It will be on a pancake course, probably Berlin, where the last six world records were set. It will be on a cool day with a tailwind. And it will have multiple paid pacemakers, as Berlin and London still do. (Boston never has and Chicago and New York no longer do.)

“You need pacemakers,’’ says Kenya’s Sammy Kitwara, the two-time Chicago runner-up who has run with and without them there. “You cannot run from Kilometer 1 to Kilometer 42 alone because of the wind. Maybe you can run the first half fast, but the second half you will be much slower.’’

Also necessary will be a top-heavy field that only the majors can command that is willing to push each other. “If one runner goes, one follows him,’’ says Ethiopia’s Lelisa Desisa, who’ll be chasing his third men’s crown in four years here. “If you do like this, they run fast.’’

They’ll also need an incentive that’s decidedly greater than victory. Boston pays the winner $150,000 plus a laurel wreath. Setting a world best brings a $50,000 bonus. But risking triumph on the chance of collecting a bonus that’s two-thirds less makes little financial sense. “Maybe if two people break two hours and get equal money then they will do it,’’ figures Keflezighi.

Breaking two hours, though, likely would bring an extraordinary level of scrutiny, particularly in a day when any athlete who goes faster, higher, stronger is suspected of using meldonium, the banned drug of the moment.

“If it wasn’t for the drugs, definitely the two-hour mark would be coming sooner,’’ reckons Korir. “You find a lot of races now looking back and restructuring, trying to go back to the original system. We have to make two steps back for us to go forward because we are pushing young men too fast too soon and they look for a shortcut. That’s what’s been happening.’’

The time to beat has been on a stalled stopwatch since Sept. 28, 2014, but the two-hour marathon, like most sporting barriers, will happen one way or another. “I think someone can do it on a treadmill sometime in the next couple of years,’’ Hall predicts.

John Powers can be reached at john.powers@globe.com.