Many 10-year-olds spend their days playing, riding bikes or sneaking as much screen time as possible. Niall McDermott spent part of his past 50 days running.

Sunday at San Francisco’s Crissy Field, Niall completed his 50th 5-kilometer run in 50 days, a challenge he used to raise more than $4,000 to research lung cancer, which his grandfather in Maryland is battling.

Niall has a mild-mannered nature and appearance. He wears glasses with black frames, has a full head of wavy brown hair and speaks in short, direct sentences. He seemed neither awed nor impressed with his accomplishment, spoke very matter-of-factly and smiled politely when people applauded him.

Niall’s 50th run took place as part of Splash and Dash, a combination 2-kilometer bay swim and 5-kilometer run. Niall skipped the swim but joined the race near the Golden Gate Bridge at the Warming Hut, starting out at a trot with a friend. He ran to Fort Point, turned around and ran back to the Wave Organ, then west again, finishing near East Beach.

As he approached the finish line, Niall displayed a burst of speed, ran between two billowing flags and stopped. As a small crowd cheered, Niall raised his right hand in the air slightly and briefly. Several of the run-and-swim athletes gave him fist bumps.

Niall said he was happy to be done with the challenge.

“It was pretty tiring,” he said, “and I was nervous for a lot of it. I thought I wasn’t going to be able to finish.”

Now Playing:

The nerves disappeared about halfway through the 50 days, he said, and he was partly inspired by the thought that he might get a trophy for his achievement. He was awarded a gift card to Sports Basement and his dad, Ryan McDermott, said a trophy will be forthcoming.

Maggie McDermott, Niall’s mother, said her son was inspired by a documentary she had him watch about the Iron Cowboy, a Utah man who completed 50 Ironman triathlons in 50 days in 50 states. His mom wasn’t sure Niall would like the video, but when it was done he told her: “I want to run 50 5K’s in 50 days.”

Niall had occasionally run with his dad but not regularly, she said. But the family set out to help him fulfill his challenge.

Ryan found scheduled 5K races, mostly on weekends. Maggie took him to the Polo Field in Golden Gate Park and to Crissy Field, both near their Richmond District home, on weekdays and he would run until his watch told him he had gone 3.1 miles (5 kilometers).

The challenge, Maggie McDermott said, was “completely self-directed,” and his parents supported it but suspected he would tire of the effort, mentally or physically, after 10 or 15 days. They also worried about injuries, but his pediatrician said that he would be OK as long as he didn’t push too hard.

“He’s been feeling great the whole time,” his mother said. “No running type injuries.”

Niall did, however, endure a few bouts of sideaches or stomach cramps, a common malady for runners. He went to see a doctor, who said he was OK and gave him permission to continue his quest.

Once it became clear that Niall was serious about making it to 50 days, the family found a charity — the Bonnie J. Addario Lung Cancer Foundation, based in San Carlos — and set up a website to raise money for the research-funding organization. Niall’s grandfather, John Eng, of Rockville, Md., is fighting the disease.

“It was important to me because my grandfather was diagnosed with lung cancer and I wanted to help,” Niall said. “I think he should be proud of me because I raised $4,000 for cancer research.”

Maggie McDermott said Eng, her father, “is touched and amazed Niall has the gumption to do this. He knows Niall loves him and that this is his way of showing it.”

Bonnie Addario, lung cancer survivor, namesake and founder of the foundation, said in a statement that she was also impressed.

“Niall is an exceptional young man and the money he is raising will help us in our efforts to end lung cancer,” she said.

Niall, in addition to a trophy, was hoping for his favorite food, cheese pizza, and a respite from running. His dad said he was going go-karting Sunday afternoon.

“I’m probably going to take a few days off,” Niall said.

How to donate Donations to Niall McDermott’s 50 5K’s in 50 days fundraising effort for the Bonnie J. Addario Lung Cancer Foundation can be made: http://bit.ly/Niall50

Read More

Michael Cabanatuan is a San Francisco Chronicle staff writer. Email: mcabanatuan@sfchronicle.com Twitter: @ctuan