Bill Gross will run his 100th marathon in Philadelphia this Sunday, thanks to a plastic grocery bag that flew under his feet in April 2013.

The 64-year-old Ridgefield resident was less than a half-mile from the finish line of the Boston Marathon that year when two bombs went off, scattering runners and observers across a frenzied city.

“It became very personal to me when I saw a shot of the clock at the finish line when the bombs went off — it had the exact time I had finished the race the previous year, in 2012,” Gross said. “It felt like they were coming after me, the everyday runner.”

Gross believes he would have matched his previous time — and been at the finish line when the bombs exploded — if it weren’t for divine intervention.

“Earlier in the race I had something happen to me that’s never happened to me in a race: I tripped over myself,” he recalled.

It wasn’t a physical breakdown.

“A plastic bag had blown across the road and tangled my feet,” Gross explained, “and I had fallen on my face. I was pretty stunned and it took me about four or five minutes to collect myself before I got up and started running again.

“That four or five minutes might have been the difference,” he added. “Someone was looking out for me.”

He returned to Boston in 2014, and finished the race he wasn’t able to complete the previous spring. It ended up being the most memorable run of his life.

“I actually always thought the Boston Marathon was a bit of a stuffy event — a badge of exclusivity for serious-minded runners, and that the route was pretty boring,” he said. “But that experience in 2013 changed my opinion of Boston, the Boston Marathon, and the sport of running. The entire city reached out to the runners. They invited us in their homes, let us use their phones. It was unlike anything I’ve ever experience. …

“I was very worried the next year would be somber, but it was anything but that — it was a celebration of everything I love about running, and it gave me a whole new inspiration,” he added. “Because that day the city of Boston took back their event and made it a celebration of running and defiance, forming a bond between the spectators and runners.”

A reformed couch potato

The road to 100 marathons started right where it will end this weekend — at the Philadelphia Marathon, where Gross ran his first race in November 2003.

A Ridgefield resident for more than 25 years, Gross grew up in the City of Brotherly Love and thought it would be the perfect site for his first attempt at the 26.2-mile challenge.

It wasn’t an exercise he ever thought he’d partake in.

“I was your classic couch potato,” Gross said. “I was the guy who made fun of friends who went for a jog at lunchtime. I never was an athlete, preferred to watch sports, not play them. …

“In the end, it served me well,” he joked. “Whenever anybody asks how my knees can withstand all this running, I answer, ‘I never used them before, so they’re fresh.’”

When Gross crossed that first Philadelphia finish line in 2003 it was only one of two big events. A month earlier, Gross had opened a new business, the Brookfield Learning Center.

This was quite a change for the former advertising executive whose career included serving as a senior vice president at J. Walter Thompson in New York.

Approaching his 50th birthday and pushing 230 pounds, it was a wake-up call for Gross to make some changes.

“I started to run a few local Boys & Girls Club 5 and 10Ks and enjoyed it, even beating a few people in my age group. Another runner suggested I try a half marathon, so I signed up for the Ridgefield Half that year and a marathon seemed possible,” he said.

“Once I crossed the line in Philly I wanted to do it again — and faster. New York and Boston were my next goals. I was hooked.”

The road goes on forever

After qualifying for New York and completing a second marathon in Chicago, an injury got in the way.

Gross was told by three doctors that he required an immediate back operation and would never run again.

He took the advice of a fourth doctor and tried physical therapy to correct a pinched nerve in his back. Six months later, he completed his first New York City marathon in what would become his personal best marathon time, 3:51.

After New York, Gross started asking other runners about their favorite events.

“I’m really lucky to have been able to go some great places I would never have gone,” he said. “Running has allowed me to experience places I have visited but from a fresh perspective and at a different pace.”

What are some his favorite memories?

“I’ve run in Paris on the Champs-Élysées where the Tour de France ends every year,” he recalled. “I’ve run in the desert outside of Las Vegas, near Area 51, in a race that started at midnight and ended at sunrise.

“My wife and I celebrated my 60th birthday with a trip to run Venice — yes — they build pontoon bridges on the canals for the day,” he added.

“I’ve also run a marathon in Rome that went through the Vatican. And that happened to be on Palm Sunday.”

Some other destinations include Berlin, San Francisco, Dallas, Miami, and Los Angeles.

“This past November I had the unique good fortune of participating in the Havana Marathon, an event no longer open to Americans.”

Pride and perspective

What is Gross most proud of?

“Finishing every marathon I started,” he said.

At first, he was obsessed with achieving time goals.

His then 11-year-old daughter, Katherine, who went on to graduate from Ridgefield High School in 2012, asked him, “Daddy, isn’t the point of a marathon to finish?”

“That put things in perspective and time goals were replaced by a new goal — finishing each race and walking away without pain,” he said.

What’s next? Marathon No. 101 in Boston, of course.

“What I tell myself is that I have to run one mile 26 times,” Gross said. “That’s how I’ve learned to push my anxieties to the side and enjoy the ride.”