KEY WEST, FLA. — The clocks Diana Nyad uses to time her training swims show that she’s a slower swimmer than she used to be. That’s only natural: At age 64, she acknowledges she is no longer the “thoroughbred stallion” she was “back in the day.”

And yet, the endurance athlete said she felt stronger than ever when she completed her successful effort to become the first person to swim 110 miles from Cuba to Florida without a shark cage.

“Now I’m more like a Clydesdale: I’m a little thicker and stronger — literally stronger, I can lift more weights,” Nyad said in a one-on-one interview Tuesday, a day after she finished her 53-hour, record-setting swim. “I feel like I could walk through a brick wall. ... I think I’m truly dead center in the prime of my life at 64.”

Nyad isn’t alone among aging athletes who are dominating their sports. Earlier this year, 48-year-old Bernard Hopkins became the oldest boxer to win a major title, scoring a 12-round unanimous decision over Tavoris Cloud to claim the IBF light heavyweight championship.

Tennis player Martina Navratilova played in the mixed doubles competition at Wimbledon in her late 40s, and hockey legend Gordie Howe played in the NHL in his 50s.

Thousands of U.S. athletes, including 60-year-old Kay Glynn, also compete during the Senior Olympics.

Older athletes tend to find more success in endurance events than power events such as sprinting and other sports that rely on “fast- twitch” muscle fibers, which are more difficult to preserve later in life, noted Wojtek Chodzko-Zajko, a physiologist at the University of Illinois Urbana-Champaign.

“She’s one of any number of people who are redefining what happens with aging,” said Dr. Michael J. Joyner, an anesthesiologist and exercise researcher at Mayo Clinic.

“If you start with a high capacity, you have some reserves,” Joyner said. “You can lose some absolute power, but what you lose in power you can make up for with experience and strategy and better preparation.”

Nyad first attempted swimming from Cuba to Florida at age 29 with a shark cage. She didn’t try again until 2011 when she was 61. She tried twice more in the past two years before beginning her fifth attempt Saturday morning with a leap off the seawall of the Hemingway Marina into the warm waters off Havana.

Nyad said her age and maturity should not be discounted when measuring her most recent success. “It’s not so much the physical,” she said. “To my mind all of us ... we mature emotionally ... and we get stronger mentally because we have a perspective on what this life is all about,” Nyad said.

“It’s more emotional. I feel calmer, I feel that the world isn’t going to end if I don’t make it. And I’m not so ego-involved: ‘What are people going to think of me?”’ I’m really focused on why I want to do it.”