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When 18-year-old Seamus Donahue ran his first marathon, its 26.2 miles stretching around the sides of Arlington National Cemetery, he knew where he would end: at the grave of his father, who died a year ago at the hands of a Taliban suicide bomber.

“I placed my medal there and I thought, he’d be really proud of me,” Seamus told Fox News. “I felt like I did the right thing. I left the medal.”

His father was the only thought on his mind throughout Sunday’s Marine Corps Marathon.

“I saw my Dad’s poster at mile 12, and I felt different instantly,” Seamus said. “Seeing his face inspired me. He was in me.”

Maj. Michael J. Donahue, 41, was killed on September 16, 2014 when a Taliban suicide bomber attacked his convoy in Afghanistan.  Donahue, a major with the Army’s 82nd Airborne Division, shared his love of running with his son.

“My dad and I first decided when I was in sixth grade to do a 5K out in Lynchburg, Va. I was a tiny little sixth grader, out of shape and all that, but we trained up together,” Seamus said. “I stuck with it, but when he was killed, I stopped running for a month. When I started again, without any training, I had significantly improved. I somehow felt like he was with me when I ran.”

Seamus, who lives in Sanford, N.C., shared a powerful image on his Facebook page of him placing his medal on his father’s headstone.

“Somehow, some way, another runner told me he was inspired, and found a way to get the race director to send me another medal,” Seamus said. “But I honestly don’t care about the medal. I just wanted to experience it.”

Wear Blue: Run to Remember, a national nonprofit running community that honors the service and sacrifice of the American military, sponsored Seamus in the Marine Corps Marathon’s 40th anniversary race.

“We sponsored Seamus and nine other gold star athletes,” Chris Bryant, Gold Star Race Program Director for Wear Blue: Run to Remember told Fox News. “They came from all over the country. They are mothers and spouses, daughters and sons, like Seamus, who lost their family members like Maj. Donahue.”

The Gold Star program is for family members who have lost a loved one while serving in the American military. Bryant explained that as far back as World War I, family members would hang a white star, representing a loved one serving the nation; but if fallen, the star would be changed to gold.

“I met a lot of other gold star families - mothers, fathers, sisters, brothers,” Seamus said. “I felt an immediate connection. You have this mutual understanding of life. I really enjoyed meeting those people.”

“We decided to reach out to the gold stars in the community to help get them through their grief,” Bryant said. “We wanted to help them to thrive and survive—to find a way through the power of running to take meaningful steps in honor of their fallen loved ones, but also to recognize their own adventure.”

Wear Blue: Run to Remember arranged and funded the travel, hotel, and entry into the marathon for the 10 selected gold star athletes who participated in Sunday’s race.

“Seamus, an 18-year-old kid, traveling for the first time on his own,” Bryant said. “He thrived. He was great the entire weekend.”

Running held Seamus and Donahue together, even when the father was overseas. Lt. Col. Jody Shouse saw Donahue just an hour before he was killed.

“I had seen him that morning before he went out on the mission,” Shouse told Fox News. “We had exchanged passions in the running community. We spoke for an exaggeration of 10 minutes and we talked about upcoming races. It’s a shame he was taken from us as early as he was. He still had a lot to give, there’s no doubt in my mind.”

Col. Trevor Bredenkamp worked closely with Donahue and shared his memories of his friend.

“There are so many people that think so highly of Mike,” Bredenkamp said. “He represents all that’s good in the Army as an officer; as a man; as a husband; as a father; and as a friend. Michael Donahue really epitomized all of that.”

“Seamus is a fine, upstanding young man. I remember seeing him and Mike at races,” Bredenkamp said. “I know how proud Mike was of the young man that he grew into and his abilities as a runner. I know they did that together.”

As for Seamus’ running career, he plans to continue with long-distance running, and hopes to, someday, qualify for the Boston Marathon.

“I feel paralyzed because I’m so sore, but mentally, I’m very much prepared for marathons in the future,” he said. “I will do the Marine Corps Marathon for my life, until I physically can’t.”