Longwill's uncle served in World War II, flying 39 successful missions. His cousin, Mike Bowen, served in Vietnam and runs with a POW-MIA flag. Longwill and Bowen plan on running a marathon together in the spring with their flags.
"I mainly did it for the local soldiers that we have still over there," Longwill said. "Maybe they'll catch it on amnews.com and say, 'Hey, we've got some people still battling for us.'
"It's a unique way of saying thanks. Those guys, we don't thank them enough, and they do it without even thinking about it. Sandstorms, bombs, everything ... we've got it easy."
Longwill also was dressed for the part. He had on a black bandanna with the words "You Are Not Forgotten" on his forehead and a picture of the World Trade Center on the back. He also had on a T-shirt from the Freedom Run in Boston with two American flags on it.
Bowen is going one step further with his running, as Longwill said he plans on running 58,000 miles with his flag - one for each name on the Vietnam Memorial in Washington, D.C.
Sunday was the first time Longwill has raced with the flag, and he said he runs about 90 seconds slower per mile with it. He finished 115th in 25:47, but estimated he'd normally run in the 21- or 22-minute range sans flag.
But the slower pace was worth it.
"There was a veteran (watching) out there, and he ran out and shook my hand, and it was kind of fun," Longwill said.
While the Constitution 5K was his first race with the flag, it wasn't his first race of the day. Longwill already had competed in the Tri for Sight, a triathlon in Lexington consisting of an 800-meter swim, a 20-mile bike ride and a four-mile run.
Longwill hopes to keep running, competing in triathlons and getting his distances up while running with the flag in preparation for his marathon with Bowen.
"I've got a long ways to go to get up to that marathon distance (of 26.2 miles)," Longwill said. "I usually run about 15 miles a week with this.
"The longest I've gone with this thing so far is seven miles. I've got to get it way up there to be able to handle that distance. He runs, with his flag, at a 4:30 (marathon) pace. I've got a ways to go, and I've got a lot of running to do."