That study, involving 1,212 subjects, has yielded three published papers since it began in 2011. Among the findings is something that would surprise even typical weekend distance runners — many of whom regard the ultrarunners as a bit of a lunatic fringe: “The injury incidence in the ultrarunners wasn’t higher than what had been reported in other studies of runners in shorter distances,” Dr. Hoffman said. Another finding was that runners who were younger and less experienced in ultramarathons — generally, races longer than the standard 26.2-mile marathon — appeared to be most at risk of injury.

Theories abound as to why: Ultrarunners, especially older ones, tend to run long, but not very fast. They know how to pace themselves. They are experienced and can marshal their resources appropriately for the big day as opposed to grinding out the miles in training day after day. And, since ultras are often held on trails, the pounding the runners’ joints endure could be less severe than it would be in a big-city marathon on unforgiving pavement.

According to Bryon Powell of Moab, Utah, who keeps statistics on ultrarunners on his website iRunFar.com, about 20 percent of all ultradistance race finishers in the United States last year were 50 and over. (Although far fewer in number than marathoners, runners who have finished an ultramarathon have risen an estimated 50 percent in the last five years, to about 25,000 to 30,000 in 2014.) In certain areas, including California and New York, the percentage of ultrarunners over 50 may even be higher.

“Ultrarunners are typically older than your average runner, no question about it,” says Mike Polansky, president of the Greater Long Island Running Club, the organizer of the 50K in which Mr. Dodson competed in March.

“It takes people who are a little bit crazy,” he adds.

Maybe so, but among the attributes that have enabled these runners, mostly men, to go the distance are some often attributed to older people: patience, caution and a reliance on well-established patterns.

“Ultrarunners stay in their comfort zone,” says Dr. Dorothy F. Scarpinato, an orthopedic surgeon who treats many runners in her practice in Melville, N.Y. “They have the same routine.”

Based on what she sees, the older ultramarathoners do not seem to suffer injury to any greater degree than those running shorter distances, an observation that squares with Dr. Hoffman’s study. For example, Dr. Scarpinato notes, the incidence of stress fractures — a common injury in runners, because of the repetitive trauma of running, especially on pavement — is low among her older ultrarunning patients.