Martina Navratilova has finally finished schooling tennis players half her age on Grand Slam courts. Now she’s gunning for the opposite age group.

AARP, billed as “the world’s largest advocacy organization for the 50-plus community,” announced that it has selected Navratilova to be the health and fitness ambassador to its 39 million members.

I doubt even Jack LaLanne reached that many people.

In a press release, AARP lauded Navratilova as perhaps the greatest female tennis player ever. It highlighted her capture of the 2006 U.S. Open mixed doubles title six weeks before she turned 50, which I can picture AARP staffers celebrating like Christmas, and her charitable and humanitarian efforts around the world. Before you ask me, let me say the release does not mention Martina’s being out and active on behalf of the gay community.

We could view this as deliberate silence, pandering to the organization’s older and thus more conservative membership. We could say this amounts to Don’t Ask, Don’t Tell, For God’s Sake Don’t Mention It.

But I see the hiring of Martina itself as an overhead smash. AARP is a huge, mainstream organization. That it wants to partner with someone the whole world knows is a lesbian is a gratifying sign of the times.

I’m sure some AARP members will cut up their membership cards into itty-bitty pieces and mail them to headquarters encased in a biblical passage or two. But the prospect didn’t scare off the AARP honchos.

Certainly I can say this about the decision – the organization chose a person who knows how annoying it is to get in shape.

In her early days on the pro women’s tour, Martina was a dumpling. Tennis journalist Bud Collins dubbed her the “Great Wide Hope.”

Oh, did things change. Aiming for new tennis heights, Martina let star basketballer Nancy Lieberman handle her conditioning. (The gossip was Lieberman handled more than that!)

In her 1985 autobiography, Martina described their first workout session. Martina was in tears, Lieberman in a fury. Martina wrote, “I was discovering true pain in my body for the first time in my life; Nancy was discovering that I did not like to exercise.”

The woman now positioned to be America’s next fitness guru used to “go to any length to avoid” exercising. As long as she still remembers that, I’m willing to give her suggestions a listen.

Martina’s eating habits were also less than stellar in her early years. She used to visit McDonald’s so often you’d have thought they were giving out Wimbledon trophies.

Thanks to her rebuilt physique and commitment to nutrition, Martina became the lean, mean, serve and volley machine who dominated women’s tennis in the ‘80s, and who managed the startling comeback in her 40s, defying time, gravity and common sense.

“We’re thrilled to welcome to the AARP family someone who has shown the world the importance of staying fit as you age,” said Emilio Pardo, AARP’s Chief Brand Officer. Sensibly, AARP has not promised that following Martina’s guidance will result in hoisting a Grand Slam trophy at age 50, 70 or 106.

In 2008, Martina will participate in AARP events, and feature prominently on its fitness Web site. The address is www.AARPfitness.com, if you’re hot to pick up Martina’s tips (as opposed to hot to pick up Martina).

For her part, Martina said her goal “is to let my generation know that though staying fit takes work, it’s just as important now – if not more – than ever before.” Sounds to me like she herself might be fitter than ever … and that’s just frightening.