Want to run the world's top marathons? Good luck getting in.

Fueled by the rise of run clubs, social media and a growing interest in wellness, marathons have boomed in recent years, from Boston to New York City.

Most people will never run a marathon. Then there’s Adrian Decunto.

Decunto has not only run the six most popular marathons in the world, he’s done them all seven times each.

“If my body allowed me, I would run a marathon every week,” he said. His dedication may be singular, but his interest in running the world’s “Majors,” as these races are known, is part of a much larger movement.

Interest in running has never been higher, fueled by the rise of marathons, run clubs and social media influencers. That running boom has pushed demand for entry into the world’s premier races to unprecedented levels.

At the center of the frenzy is the Abbott World Marathon Majors, a circuit of seven of the sport’s most coveted races: London, New York, Berlin, Chicago, Tokyo, Boston and, most recently, Sydney. For many runners, completing all six of the original races has become the sport’s ultimate achievement, a feat that comes with a giant doughnut-shaped “six-star” medal and bragging rights for completing the equivalent of the “Amazing Race” for marathoners.