Liliana Retelny, 47, practices rowing at the Miami Beach Rowing Club in Miami Beach, Fla. Retelny is a boomer who took up a competitive sport at a mature age. She has competed in various international competitions and has already won silver and bronze medals in various games.

MIAMI — The sky is the color of freshly brewed coffee when Liliana Retelny slips her 27-foot shell into the still waters of Miami Beach's Indian Creek and begins her daily three-hour routine. She rows. She rows as the rising sun stains the clouds, as students practice with their crew teams, as the sounds of a waking city begin to fill the air.

Retelny, 47, is practicing to compete against rowers two decades her junior. The Aventura, Fla., psychotherapist already has won two silver medals in the Central American Games, placed 20th in World Cup competition and second in her division in Israel's Maccabiah Games. All this in a sport she took up only four years ago, when her daughter was rowing for her high school team.

"I love it," said Retelny, who competes under her maiden name, Boruchowicz. "For me this is not work. It is not a matter of discipline. When I'm on the water, I'm the happiest. I feel alive and young."

The Costa Rica native is part of a growing cadre of baby boomers who seek the proverbial fountain of youth in swimming pools, on running tracks and in the gym. Many have taken up sports — even extreme sports — in mid-life, pursuing fitness not only to look good but to feel good.

"Boomers have always appreciated being physically fit, and they're not about to let go of that active lifestyle," says Kara Thompson, spokeswoman for the International Health, Racquet and Sports Club Association (IHRSA). "They want to stay healthy. They exercise because it makes them feel better."

Jim Loretta, 63, had always run to keep in shape. It was a form of exercise he could do around his Kendall, Fla., neighborhood or on a hotel-gym treadmill when he traveled for his firm, Loretta Marketing Group. Then, about 20 years ago, he got to talking to two marathoners. "It sounded like something I wanted to do," he says.

When he finished his first half-marathon in November 1992, his wife asked, "Have you had enough?"

He was just getting started. By January 1994, he had run four marathons at progressively faster clips. To train, he was running 40 to 60 miles a week. Bitten by the competitive bug, he moved on to triathlons — swimming-cycling-running competitions — and then to the Ironman, a grueling endurance event that combines a 2.4-mile swim, 112-mile bike ride and 26.2-mile run.

Training means discipline and sacrifice — up at 4:15 every morning to run, swim or bike. Weekends are more intense: a 12-mile run on Saturdays and a 60-mile bike ride on Sundays.

By 2008, with four Ironman competitions to his credit, "I was in phenomenal shape," he says. "I felt really good."

While Loretta's training regimen is uncommon, his commitment to fitness is not. His generation, he says, "wants to keep in shape. They know exercise is good for them."

There are no statistics on the number of boomers who exercise on a regular basis, but experts say they probably are more physically active — at least recreationally — than past generations at middle-age.

Industry surveys find that boomers are the fastest-growing segment of the health-club population, with those 50 and older accounting for 23 percent of members, says IHRSA's Thompson, whose association represents many of the country's 30,000 gyms.

"Boomers are much more into exercise than their parents were and even more than their children are," says Santiago Matute, a certified professional trainer who works at three Miami-Dade gyms. "And they're very knowledgeable about their bodies and nutrition. In general, they're taking better care of themselves."

Renee Grant, 53, took up running 10 years ago, after a divorce. She worked up to a mile on a neighborhood track, then joined a runners group in Hollywood and built up to five miles.

"At first it was a mental escape for me," says the Cooper City, Fla., resident. "It made me feel so good to be out. It was quite an adrenaline rush."

A year later, she ran her first 5K in Weston, Fla., finishing in the middle of the pack. She completed her first half marathon in 2003 and her first full marathon in 2004. Since then she has run two a year as well as shorter races about once a month. In last month's Boston Marathon, she ran 3:40:41, beating her previous time by five minutes.

"It's like an addiction," Grant says. "I have to do it. If I don't run, I become grouchy and irritable."

Running became such a big part of her life that she converted her dancewear store in Davie, Fla., to Runner's Depot, now a four-store South Florida chain. She's happy to tout the benefits of the sport to customers.

"After a recent bone density test, I was told I had the bones of a 25-year-old, and I can feel it. I have more energy than I've ever had in my life."

Studies show that boomers exercise differently — and more consistently — than their younger counterparts, with 55- to 64-year-olds showing up at the gym an average of 112 days a year.

"They lead the pack," says IHRSA's Thompson. At Anytime Fitness in Hollywood, Fla., owner Guy del Borrello, calls his boomer clients "more dedicated and serious. They give more importance to health."