Truro, N.S., runner Jody Mattie is hoping to find some good out of a terrible tragedy. 

He was in the first aid tent at the finish line of the Boston Marathon in 2013, the day of the deadly bombings.

Now, he's raising money for a charity called 50 Legs that helps amputees.

"It was a tough race for me," Mattie said Tuesday, "I was sick beforehand so it was a gruelling run."

As he was being treated for dehydration, he heard the explosion and chaos at the finish line.

'It's all I could think of'

Celeste Corcoran

Celeste Corcoran lost both legs in the Boston Marathon bombings in 2013. (Robert X. Fogarty)

"Being in the medical tent, I think I was just anxious. I was more worried about my children and my friends who were in the waiting area for me. I was worried that they were worried not knowing where I was in relation to the bombs."

All of the victims had to go through the medical tent to get to the ambulance. They passed close to Mattie on their way to triage. What he saw left a mark on him.

"For close to a year, you know, every day, every night, it's all I could think of," he said.

Mattie says he was invited to go back to the Boston Marathon the year after the bombing and that's how he became friends with Celeste Corcoran, who lost both legs in the blast.

Corcoran's daughter Sydney was critically injured and almost died.

A 'comeback run'

Celeste and Jody

Mattie said Celeste Corcoran asked him to run on her behalf in next month's race. (Carmen Acabbo)

Mattie said Celeste Corcoran asked him to run on her behalf in next month's race and raise money for the charity that provided her with her prosthetic legs and continued support.

"I'm pretty excited about it actually. It's kind of a comeback run for me," he said, "I've a little over a year off due to an injury so this is my first race since."

He said Boston 2016 is also a comeback race for his friend Tracy Cohen-Peranteau, who was in a car accident seven months ago. It was Cohen-Peranteau who first introduced him to Celeste Corcoran

"There's a group of us raising money for 50 legs, so we'll all run together," he said.

As of Tuesday evening, Mattie's fundraiser raised about half of the $5,000 goal. 

This year's Boston Marathon is Monday, April 18