Lelisa Desisa, Eliud Kipchoge and Zersenay Tadese are all part of Nike’s project to break the two-hour marathon barrier. (Courtesy of Nike)

Ed Caesar knows more about the two-hour marathon than most. The British author spent four years researching his 2015 book, “Two Hours: The Quest to Run the Impossible Marathon,” reaching a consensus with the experts he interviewed that the feat would happen “sometime around the next 20 years.”

That’s why he was taken aback by the timeline for Nike’s secret project announced Monday, dubbed Breaking2, to train the first sub-two-hour marathon runner. Asked by Wired magazine this fall to cover Breaking2, Caesar figured that the sports-apparel giant was planning to make running history by 2025, maybe 2020. Instead, the project’s race attempt will occur this spring.

“It made no sense to me,” Caesar said Monday by phone from the U.K. “I just had so many questions: how they were doing it, who is involved, how long are they going to do it.”

On Friday morning, the Wall Street Journal reported that Adidas has also been working for at least two years on a similar project and has produced a prototype of a running shoe with the hopes of accomplishing the feat on an established course. Adidas sponsors the current world-record holder, Kenya’s Dennis Kimetto, 32, who set it with a time of 2:02:57 at the 2014 Berlin Marathon.

The two-hour marathon is running’s Mount Everest, a once seemingly unattainable goal that runners have nonetheless long been aiming to conquer. Enough have come close that experts believe it’s a matter of when — not whether — a marathoner can cover 26.2 miles in under two hours. Still, few believed it could happen as quickly as Nike’s ambitious projection.

“Many consider this feat impossible,” the company said in a news release Monday. “However, that challenge is exactly what drives Nike; the impossible is an opportunity to envision the future of sport.”

Nike began working on footwear specific to the marathon in 2013 and turned its efforts to breaking the two-hour barrier a year later. The project is led by a 20-person team that includes designers, engineers, coaches, nutritionists, psychologists and physiologists. The race in the spring will not be eligible for a world record, according to the Wall Street Journal’s Sara Germano, and will likely aim to eliminate environmental factors and include pacers to help the runners — a plan that has been referred to by runners and coaches as a “moonshot marathon.”

Rio Olympics marathon gold medalist Eliud Kipchoge of Kenya, 32; Lelisa Desisa, 26, of Ethiopia; and Zersenay Tadese, the 34-year-old Eritrean world-record holder in the half marathon (58:23), have all signed up for Nike’s project. The specific date and location of the race will be revealed next year.

“I know one day [two hours] will be broken,” Tadese told Runner’s World through an interpreter. “I want to be a part of it.”

Toward the end of his book, Caesar details the complexities of a moonshot marathon. It would require athletes who choose to forgo competing in lucrative races, a company with plenty of financial muscle, a window of a few days of potentially ideal running weather (windless and chilly) and a string of pacers to help the runners along the flat course.

These are ideas Nike could make a reality, Caesar realized as he toured the Nike headquarters last month in Beaverton, Ore., while on his assignment for Wired.

“I was very impressed by the seriousness in their approach to this,” Caesar said. “Whether or not they succeed is one thing, but they’re taking it seriously as something academic and practical to achieve.”

Michael Joyner, an expert in human performance at the Mayo Clinic in Rochester, Minn., published a paper in the Journal of Applied Physiology in 1991 stating that it might be physiologically possible for a 1:57:58 marathon runner. Earlier this year, though, Joyner, 58, backtracked, writing in Sports Illustrated that “if I am lucky, a legitimate sub-two-hour marathon will happen in my lifetime.”

Some have been even more optimistic. David Martin, a statistician and running historian, told Runner’s World in 2012 that sub-two would occur in 2015. Douglas Casa, an American sports scientist, told Caesar that he believes it would happen within a decade.

But to others, dropping nearly three minutes off the world record in the marathon in such a short amount of time is difficult to fathom.

Olympian Meb Keflezighi, one of the United States’ top marathon runners, won the silver medal at the 2004 Athens Olympics and ran a personal-best time of 2:08:37 at the 2014 Boston Marathon. The 41-year-old estimated that he was in shape to run in the 2:05 range during his prime and questioned whether runners today are ready to break the two-hour barrier. Of the three Nike runners, Kipchoge has the fastest marathon time, 2:03:05, set this year at the London Marathon.

“There should be nobody at the elite level that should [run a] three-minute [personal record],” Keflezighi said Monday by phone. “At New York City, I ran like 2:09 10 times. Why? Because it’s so hard to take off seconds — not minutes, but seconds.”

Nike understands the doubt and, in response, points to 1954, when Roger Bannister became the first man to clock under four minutes in the mile. It was an athletic achievement once thought to be humanly impossible. Nike is hoping that this spring’s race will be a similar moment.

“I think it’s a long shot, but I think it’s not impossible,” Caesar said. “I don’t know how you would phrase it as a fraction or as an odds, but I don’t think it’s impossible that they’ll do it.”

This post has been updated with news of Adidas’ plan.