Terrorized by the deadly events of that day, he has spent the last year struggling with survivor's guilt.
He was so traumatized, he thought he was never going run again, even questioning a lifetime of inspiring others to give back and set goals.
"Just before I was going to finish at the turn onto Boyles Street, there was an unexpected finish and we watched those bombs explode," Grogin recalled.
Running for Paul Newman's Charity that day, the 28-time Ironman finisher spent more than 30 years racing and had a record of always finishing his races. But with a sudden blast, that all changed.
"They put up barricades very quickly between you and the finish line and they said the race is over," he said.
He considered himself done too. Left questioning why he was even in a marathon, he wondered what it mattered if he raised any money at all.
"It all seemed so ridiculous that for the sake of running 30 seconds faster than in another race," Grogin said. "People were dying. I was positive that I was done in that moment to not do any more racing."
But the idea of quitting didn't last for long. Knowing that all eyes would be on Boston this year and remembering the kids who depended on charity money for camps, he organized Team Grogin to plan for the race of a lifetime - running 32 miles a day from Franklin Lakes, New Jersey to the start line of the Boston Marathon.
While it is bound to be an emotional trek, it will also certainly be a physical challenge for the 59-year-old chiropractor. The course he's taking is 250 miles in total and has a 16,000 elevation gain with rolling hills all the way to Massachusetts ? all before the 26.2 mile marathon.
"The greatest challenge is just staying on course and remembering that we're running for those kids and when we remember that it's pretty effortless," Grogin said.
When Team Grogin gets to the Boston finish line, they will be prepared with a message to the world.
"We didn't give up and we didn't quit. We didn't let somebody else decide what was right or wrong," he said.
Grogin hopes that the miles turn into dollars and he'll have a big check for SeriousFun, the new name of Newman's charity.