The fitness guru, whose first exercise video in 1982 sold 17 million copies -- more than any other home video -- drew thousands to the Georgia Dome for the event, which she hosted and created to promote good health.
"I waited until my 40s to start getting healthy and look at me now," says Fonda, who hopes to make World Fitness Day -- which she calls "the exercise Woodstock, without the drugs and rain" -- an annual event.
"Forget what it does to your body. Look what it does to your mind. You feel more alive."
Fonda originally took up exercising following a foot injury during the filming of 1979's "The China Syndrome."
She went on to release five workout books and 25 workout videos, the most recent in 1995.
Though she's now the very picture of good health, the sleek star underwent hip replacement surgery five years ago and had knee replacement surgery last year.
"It's never too late to work out and it's never too early," she says. "I want to encourage everyone to get up and exercise."
That's why she created World Fitness Day, which benefits the Georgia Campaign for Adolescent Pregnancy Prevention and brought out fellow fitness instructors including Richard Simmons and Billy Blanks.
"People are not being told how to eat properly or how to exercise," Fonda says. "We are far too sedentary. We spend most of our day sitting. Get up!"
For more fitness fashions, check out our gallery of what stars wear to the gym.