The powerful explosion just short of the finish line knocked him off his feet and changed his life.
The photograph of Iffrig taken moments after the second of two blasts - he was lying in the street, surrounded by three Boston police officers, one with weapon drawn - ended up on the cover of Sports Illustrated magazine and became the lasting image of the horrific bombings on April 15, 2013.
Iffrig was able to get to his feet and finish the race, but he came away with an emotional trauma that is, in part, why he is not in Boston today.
"I just haven't been ready yet to get out and do it again," he said.
Iffrig, now 79, also suffered two apparently permanent physical injuries. He has significant hearing loss in his left ear (the side nearest the explosion), and now wears hearing aids in both ears. He also has minor muscle damage in his right thigh, the result of his fall.
And even though he continues to train ambitiously - 50 miles in a good week - he has not run a marathon since Boston.
"I just haven't had the need to do it, I guess," he said. "I don't know what it is really. I just haven't had the notion."
Though he considers himself extremely lucky - "That's one of the first things I thought of, even when I was lying there," he said - Iffrig still has difficult memories of that terrible afternoon. Likewise Donna Iffrig, his wife of almost 60 years, who was in their hotel room on race day, listening to news reports of the explosions and not knowing if her husband was OK, injured or dead.
"When I got back (to the hotel room), she was very upset," Iffrig said. "So we hugged and talked about it and cried a little bit. It was very emotional for me. I'd tear up when I even thought about it. It was just such a close call."
The couple remained in town for four more days, and their grief was offset by the remarkable kindness of the Boston people, many of whom recognized Iffrig and came over to wish him well. On the trip back to Seattle, the airline flight attendants treated the Iffrigs like royalty.
It was disappointing, though, that Boston race officials never reached out to him in the ensuing weeks and months. Despite his injuries - and an Everett doctor even sent organizers a letter on Iffrig's behalf - "I never heard a thing from them," Iffrig said. "I didn't want to make too much of a fuss out of it because I really wasn't hurt that badly. But it just kind of left a bad taste in my mouth.
"They could've done a little more for me," added Iffrig, who paid $3,500 for the hearing aids out of his own pocket.
In the past year, Iffrig has been asked to tell his story dozens of times. Media in Western Washington have been frequent callers (including The Herald), but there have also been interview requests from elsewhere in the country and even around the world.
He returned to running almost immediately, including the 12K Bloomsday Run in Spokane just two weeks later. Race officials there rolled out the red carpet, putting the Iffrigs up in a posh downtown hotel for several days. Several runners, including about 20 visiting Kenyans, tracked him down to get their pictures taken with him.
Friends urged him to enter this year's Boston Marathon, but after running the race in 2012 and 2013 "I didn't really want to go back again," he said. "Maybe I'll go next year, because I'll be 80 then. But I haven't made any decision yet. It's too early. We'll see."
But he is by no means done with marathons. Iffrig, a 1953 graduate of Everett High School, expects to run two this year, beginning with Spokane's Windermere Marathon on June 1. In October he also plans to run the Skagit Flats Marathon in and around Mount Vernon and Burlington. By then he will be 80.
Trying as the last year has been, Iffrig sees the months ahead and the two coming marathons as a chance to leave behind the awful memories of a year ago.
"And I'll be ready," he said.