Road races are also experiencing competition from things like studio classes, group yoga and CrossFit. High fees for races have also become an issue. In 2006, the average five-kilometer race cost $13.50, according to Road Race Management, an industry publication. Now the average is $34. In 2006, the average marathon cost $69.97. That’s now $123.

In a 2017 survey, Running USA found that 20 percent of runners expected to decrease their race participation. Half of the runners in the survey said they thought that races were too expensive and that cost was a top 10 factor in deciding whether or not to register for a race. The survey also indicated that six in 10 runners would participate in more races if fees were low.

Phil Stewart, the president of Road Race Management, said that prices had grown so quickly because they could — people kept paying them. But now races are starting to see price resistance.

“Back when you could enter a road race for $10 and you could enter a marathon for $25, the sport really had no appeal or very little appeal for for-profit businesses,” Stewart said. “But then we moved into an era where people would pay $85 for a half and $135 for a marathon. That’s when you really had all the for-profit groups, and it just transformed the model.”

Stewart specifically cited the Rock ’n’ Roll Marathon race series, which put a heavy focus on the social experience of running events and charged high prices for it. (Fees for races in its 2017 Las Vegas race series are $79.99 for the 5K up to $179.99 for the marathon.)

Jean Knaack, the executive director of the Road Runners Club of America, said that running is “at the plateau.”

“While we’re seeing the decline on the event side,” she said, “people are still staying active in their clubs.”