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 says 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 told The Associated Press in a one-on-one interview today, 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.

Glynn, of Hastings, Iowa, has won six gold medals in pole vaulting at the Senior Olympics and set a new pole vaulting world record for her age in the 2011 National Senior Games.

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.

But just because Nyad was swimming rather than pounding her joints against the concrete doesn’t mean she didn’t achieve a remarkable feat, Chodzko-Zajko said.

“This ultra, super-length swimming is brutal regardless,” he said, adding that another reason athletes are able to endure is because they often train smarter and have a mental concentration that is well honed over decades.

“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. She paused occasionally for nourishment, but never left the water until she reached the white sand beaches of the Keys and waded ashore.

Nyad says 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.”

Australian Susie Maroney successfully swam the Straits in 1997 at age 22 with a shark cage, which besides protection from the predators, has a drafting effect that pulls a swimmer along.

In 2012, 49-year-old Australian Penny Palfrey swam 79 miles toward Florida without a cage before strong currents forced her to stop. This June, Palfrey’s countrywoman Chloe McCardel, 28, made it 11 hours and 14 miles before jellyfish stings ended her bid.

Nyad admitted Tuesday that she was glad when McCardel didn’t make it before she had had a chance to, but she did add, to laughter from her team, that “I didn’t want her to get bitten by jellyfish or die or anything.”

Nyad said Tuesday that that she wasn’t finished with marathon swims. She plans to swim for 48 hours straight, accompanied by celebrities swimming laps alongside her, in a specially designed swimming pool that will be erected in New York City next month to raise money for Hurricane Sandy survivors.

Although the swimmer insists she wasn’t trying to prove anything as a 64-year-old — “I didn’t do this because I was in my 60s. I just happened to be in my 60s,” she says — she acknowledges that her success is having an impact, “not just on people of my generation but on younger people.”

“I have a godson who’s 14 and he texted me yesterday and said, ‘I’m never in my life again going to call someone in their 60s old. It’s over. You just proved that youth doesn’t have anything to do with age.'”

And at one point during her AP interview Tuesday, the bronzed, muscular athlete couldn’t resist sharing a message of encouragement and solidarity with those of her generation:

“Baby Boomer power!” she declared, with a triumphant fist pump.