Imagine yourself across Boylston Street from the first explosion last April, near the finish of the 117th Boston Marathon.

Confusion reigns after the first blast. Twelve seconds later, another bomb explodes.

Do you run for cover or wade into the chaos, not knowing whether a third explosion is imminent?

John Mixon of Ogunquit was there. He looked at his friend Carlos Arredondo, wearing a white cowboy hat and clutching a small American flag.

“We both knew what we had to do,” said Mixon, who had been tracking the five runners raising funds for Maine’s Run for the Fallen, the organization he co-founded and continues to lead.

“You just had to be a man, not a hero. I did what any man would have done.”

There is no boasting in Mixon’s tone, no triumph. Three people died. More than 260 were injured. Rather than flee the confusion, he tore down fencing and helped load victims onto gurneys. Mixon’s actions helped to save lives that afternoon. In the year since, he has been faced with tragedy and adversity of his own – including a heart attack in September that left him on life support for eight days. But on Monday, he plans to be back at the finish line of the 2014 Boston Marathon.

“It’s amazing what he’s been though,” Arredondo said of Mixon. “He’s a very strong man.”

A veteran of the Vietnam War whose life’s work is honoring soldiers who have made the ultimate sacrifice for their country, Mixon, 61, was hailed as a hero. The Red Sox, Bruins and Celtics invited him to games and celebrations, including the World Series parade and Opening Day.

In retrospect, Mixon said, one of the biggest reasons he didn’t hesitate to help, despite not knowing whether a third bomb would explode somewhere nearby, is that he knew his wife and daughter were safe.

Linda Mixon was back home in Ogunquit, watching the events unfold on television. Erica Mixon, a 2011 Wells High graduate now in her junior year at Emerson College, had planned to join her father at the finish line but because of an internship interview, she remained in her apartment five blocks east of Copley Square.

“He called me right away,” Erica said, “and said something along the lines of, ‘There’s been a terrorist attack. Don’t leave your apartment.’ And then he pretty much hung up.”

Meanwhile, Mixon and Arredondo sprang into action. Mixon tackled the fencing built to separate runners from spectators, but now separating victims from first responders.

“It’s actually a snow fence wired onto scaffolding with a mesh fence on the runners’ side,” Mixon said. “It’s designed for crowd control, so there are no breaks in it, no entrances. It ran all the way down to Hereford (Street).”

Mixon began pulling at the fencing, ripping it apart, and soon was joined by members of the National Guard who had carried 40-pound rucksacks from the race’s start in Hopkinton, Mass. Arredondo, whose son Alex was killed in Iraq in 2004, had already vaulted the fence and was attending to survivors.

The Associated Press photograph of Arredondo in his cowboy hat helping to rush legless bombing victim Jeff Bauman in a wheelchair toward medical attention became an iconic image of the tragedy’s immediate aftermath.

When things settled down slightly, Mixon walked over to where he had dropped two yellow bags he had brought back from Hopkinton that morning, containing changes of clothing for Run for the Fallen runners. He stopped suddenly.

“A Boston police officer put a gun to my head and cocked the hammer back,” Mixon said. “He said, ‘What’s in those … bags?’ I stood up and said, ‘I’m just trying to help.’ He said, ‘You want to help? Get the (expletive) out of here!’ ”

Mixon complied. Eventually he called his daughter again to assure her he was safe, and he and Arredondo went to her apartment to clean up.

“It was definitely a strange sight,” Erica said. “My dad had dust on him and a little bit of debris, but he didn’t look as bad as Carlos did. His shirt was tattered and torn and there was blood all over it. They were both in a state of shock.”

In the year since the bombing, Arredondo became the face of a city’s resilience, the strong-jawed hero in the white hat, and has appeared at Fenway Park and TD Garden and at many other public gatherings, often with Bauman, who is engaged to the woman he had come to cheer, now expecting their first child.

And Mixon?

He, too, has accepted invitations to sporting events and appearances with first responders and survivors, including an 11-day Boston Heroes cruise on the Seine River in France.

“There were some fun moments, but it was extremely emotional,” Linda Mixon said. “I believe I cried every single night at dinner.”

Mixon also endured the death of his father in July; the near-fatal heart attack in September while vacationing on Prince Edward Island; his mother’s deteriorating health leading to a move to assisted living; and the deaths of two beloved family pets, a chocolate lab named Nike and a border collie named Quickie.

Nike died just before the trip to France. Quickie died in the back seat of the car at a Prince Edward Island gas station while paramedics were working on Mixon in an ambulance. A friend had called 911 while Linda was inside fetching aspirin for John to chew.

“I wish I was making this up,” Linda said. “Right before he got in (the ambulance), he passed out.”

Mixon and Arredondo plan to watch this year’s race from the same place on Boylston Street in front of Boston Public Library. Mixon will be watching for a dozen runners involved with Run for the Fallen fundraising, including two who are running in honor of Alex Arredondo. Run for the Fallen is held to honor Maine soldiers who have died since the Sept. 11 terror attacks and to assist their families.

Mixon’s first instinct, in the aftermath of last year’s race, was to run Boston himself this year, something he has done more than 20 times, often with race director Dave McGillivray in his post-marathon evening tradition. “Great guy, great friend, very generous,” McGillivray said of Mixon.

The heart attack scuttled that plan. Mixon spent time at two hospitals on PEI and a third in Saint John, New Brunswick, all on the day of the heart attack. Another week passed before he was stable enough to be transported to Boston, then a few more before he was moved to a rehabilitation facility in Portsmouth, N.H.

By Thanksgiving, though, Mixon was back on the roads, shuffling through a 5-kilometer Turkey Trot while following his doctor’s orders to keep his heart rate below 125 beats per minute, down from his usual race-pace exertion of 165. He took part in other road races, including a Run for the Troops in Andover, Mass., a New Year’s event in Lowell, Mass., and Saturday’s Boston Athletic Association 5K.

“Ninety-five percent of the people who have that heart attack, even in an urban area, don’t make it,” Mixon said. “So God’s got some kind of plan for me. I just hope it’s not another bombing.”

Frolicking about the Mixon household is a new puppy, a black lab.

“We call him Chance,” Linda said. “For a second chance at life.”

Glenn Jordan can be contacted at 791-6425 or at:

Gjordan@pressherald.com

Twitter: GlennJordanPPH