"People come up to me now and start talking to me about someone in their family, or a friend, or a loved one that has some type of leukemia." He said the experience has opened up a new world to him.

Sports psychologist Jack Llewellyn says that very few professional athletes want to think about coming to the end of their careers while they're still playing. "As a consequence, very few of them are prepared for that next chapter."

Finding a passion off the court while she was still playing in major tournaments helped Martina Navratilova. The tennis superstar, now 55, played professionally well into her 40s. She said that after retiring, "you become irrelevant really quickly."

But because she never defined herself solely through tennis, she was able to accept the transition.

"My sense of self-worth did not depend on winning matches," she said.

Navratilova still is committed to keeping fit: she runs with her dogs, skis, cycles and plays hockey. "And of course, I play tennis."

Navratilova serves as fitness ambassador for AARP, which she says she loves. And she wrote "Shape Your Self: My 6-Step Diet and Fitness Plan to Achieve the Best Shape of Your Life."

Her advice to other athletes, professional and amateur? Play a new game when you get older.

"Find another sport that you can really improve at, that you can get excited about, and have fun," she said. Athletes can still satisfy their competitive drive, without comparing their current game to how they used to perform when they were younger.

Like Navratilova, Kevin Willis played professionally much longer than most others in his sport. His career with the NBA stretched into his 40s. But he still didn't want to retire.

"If it wasn't for the simple fact that I'm older, I would (still) be playing," he said. He finally stopped because he says he didn't want to wear out his welcome.

Willis spent half of his prolific career playing for the Atlanta Hawks. The president of the team, Bob Williams, acknowledged that most NBA athletes don't want their playing days to end.

"It's hard to give up the adrenaline rush," he said. He noted that Willis is exceptional in carefully orchestrating his next step after the NBA, and other players could follow suit.

"He's leading by example," Williams said.

Willis already had a post-NBA pursuit lined up when he stopped playing for good five years ago. He started a clothing brand, Willis and Walker, back in 1988. It caters to men who are 6-foot-3 inches and taller, a demographic the 7-footer understands very well.

Speaking from his boutique in Atlanta, he told CNN: "The relationships that I built over those 21 years from basketball, I tapped into ... to help me build this." His clients include former and present professional athletes.

Having two decades worth of NBA earnings helps Willis to finance the endeavor. He says he has poured more than $1 million of his own money into the brand.

Willis turned 50 this year, but he can still carry his weight -- and then some. He says he can still bench press 315 pounds, just as he did when he played professionally. But now, instead of lifting one set of that weight, he completes five sets of five or six repetitions.

"And how did that happen?" he said. "I don't know, man. It's just in the genetics, I guess."

Without the rigors of the NBA game schedule, Willis has more time to train -- and to reflect on his recent milestone birthday.