One day in 1989, Joanna Rohrback was strutting along the Hollywood Beach boardwalk near her Florida home.“A really good song came on my Walkman,” says Rohrback, who soon found her arms and legs bouncing to the beat.As she began to trot and then gallop, she recognized what had been missing in her exercise routine: some horsing around.That epiphany led her to create Prancercise, a fitness concept inspired by equine movements that took the Internet by storm in May.Millions have watched Rohrback hop around in tight white pants and ankle weights, explaining the four basic steps, in a YouTube video.It’s a nice introduction, Rohrback says, but anyone who really wants to “cut the noose and let it loose” needs more instruction.As she explains in her book, Prancercise: The Art of Physical and Spiritual Excellence(WingSpan, $9.95), the overall Prancercise philosophy goes well beyond footwork and shadow boxing. This style of fitness is a celebration of self-expression. She doesn’t expect anyone to mimic her exactly, but to find inspiration from her movements.“If you were to look at horses, they spring off the ground, using all four limbs and their heads,” says Rohrback, who has always had a fascination with the animals. “But every horse moves a little differently, too.” Rather than follow a choreographed routine — “This is not Zumba,” she says — Prancercisers are encouraged to explore what feels right for them.There’s no need to use ankle weights, and people can choose their level of intensity. They can do it anywhere. (“I’ve done it on mountains and in cornfields,” she says.)All that matters to Rohrback is that students tap into muscles that are usually ignored and improve their range of motion by opening up their hips and shoulders and blowing off some steam.Rohrback credits that combination with helping her reclaim her life last year after more than a decade of setbacks. She developed Prancercise and penned her book in 1994 but couldn’t find a publisher.Then she was forced to shelve the project to be a caregiver for her mother, and eventually she had health troubles of her own. For years, Rohrback was Prancercising for only 5 minutes a day.But in July 2012, she decided to give the program another shot.“I thought, ‘I’m turning 60 years old. It’s time to do this,’” says Rohrback, who self-published the book and began upping her Prancercise workouts.By the end of the year, she’d Prancercised a 5K and filmed that viral YouTube video that went live on Christmas Day.It has been a wild ride since Rohrback became a Web sensation. She has been deluged with requests for appearances and classes and even a reality show. Everything is building up to her vision of a nation of Prancercisers, says Rohrback, who would like to see a network of studios where people can experiment with moves together.“It takes practice to fully enjoy it,” she says. “When you’re happy with your leg work, work on your arms and head.”Not surprisingly, the Internet is full of Prancercise parody videos.Although the moves Rohrback shows off in John Mayer’s Paper Doll lyric video are billed as “Advanced Prancercise,” there’s no such thing — yet.“I’ve been working on something that’s a little more springy,” Rohrback says.A few million people are waiting for that video.