The 68-year-old Loma Linda University librarian crossed the line at 2:44 p.m. and lingered near the finish area to recover - drinking water, eating energy bars and joking with fellow runners.
Nearly six minutes later, he heard a loud boom.
"It sounded like someone set off a Fourth of July cannon," said Johns on Tuesday, while he was still in Boston. "But then when I heard it echoing down the street, when I felt it reverberating, I knew it was a big explosion."
Seconds later, another bomb detonated.
"Immediately after I heard the second one, I turned to the runner next to me and said 'terrorism,'" Johns said. "That seemed to be everyone's response."
Officials promptly started tending to the injured and moving runners out of the area.
"Policemen were rushing past us through the crowd, running," Johns recalls. "The police response was incredible."
Johns, who had run the race solo, was by himself.
"I had two or three kind pedestrians offer to help me because I was all alone and shivering a little," he said. "The kindness of the Bostonians is something that should be noted."
He walked about two blocks to the buses, where he had hoped to retrieve his belongings. But he came across what he described as a frantic scene.
"People were desperately torn between fleeing the area and getting their race gear," he said.
Someone pulled his gear from the bus and handed it to him.
"I got out of there as fast as I could," he said. "There was chaos everywhere. Firetrucks going through, police cars, ambulances."
"Everyone that had a cell phone was on it. We couldn't get through, I couldn't call my wife or my friends and relatives."
It took about half an hour, but Johns eventually found his friends who had come to pick him up after the race. They drove him back to their home, which was about a mile out of the city.
"When I finally got a hold of people, everyone was frantic," Johns said.
"My wife knew I would be starting the race about 10:45 am and that I was hoping to run it under four hours, so when she calculated that at the end she figured I was right there when the bombs went off. You can imagine her reaction."
Johns, a seasoned marathoner, ran the Boston Marathon four years ago. When he qualified for this year's race he was excited to go back.
"It's the most beautiful time of the year because spring starts late in Boston," the flowers are blooming, the weather is warm, Johns added.
He doesn't know yet if he'll be running in any future Boston marathons, but he will run Tuesday with his local group - the Loma Linda Lopers.
"I'm sure I'll be talking about this with them," he said.