What started as a buddy’s birthday celebration snowballed into a full-blown business for John Vergato and Eric Dunphy.



Two weeks ago, Vergato quit his day job to concentrate on “Great Boston Experiences,” a start-up company that hosts events like this Saturday’s “Great Boston Beer Marathon” in the Fenway area, one of the biggest bar crawls in America.



Now in its sixth year, the beer marathon originally started as a pub-crawl-style birthday party for a co-worker.



Through Facebook and word of mouth, Vergato and his friends got more than 750 people to turn up.



This year, they are expecting close to 3,000.



“Three of us started at the same company doing medical software, and we were cubicle monkeys,” said Vergato. “We started talking about what we should do for a birthday and it just spiraled really unexpectedly.”



For an advanced $25 ticket, the Boston bash this weekend gets partygoers a discount on food, a swag-bag full of gifts, a gold event shirt, a Viking hat and a day pass to 26 bars.



From Kenmore Square to Allston, boozers can hit all the spots along the route.



Vergato said since quitting his job to concentrate on the party business, he now plans on branching out to launch similar gatherings in New York City.



“It was just something we wanted to do for fun,” he said. “Now I tell my parents I run beer marathons.”