"It's scary, and you need to go with really experienced people."
Photo: Michael Simon

Katie Holmes recently said she's in the best shape of her life, thanks to her role in the upcoming thriller The Doorman. But the actress and mom has long made a conscious effort to make physical activity a part of her everyday routine.

"I try to stay in shape," she told us at Westin's Global Running Day event where they announced their global collaboration with Charity Miles, a company that allows you to earn money for your charity of choice while working out.

"I ran the NYC Marathon in 2007, and I've been running since I was a little girl. My family runs," Holmes continued. (Related: Running Tips From Katie Holmes' Marathon Trainer)

Over the past couple years, Holmes has been dipping her toes in a whole new spectrum of workouts that challenge her body in different ways. "I don't run every day," she says. "I also do yoga, cycle, and lift weights."

Nearly six or seven months ago, she took up boxing too. "It's a really fun, empowering workout," she says.

While Holmes is no stranger to pushing her body to its limits, there's one fitness adventure that challenged her the most: scuba diving. "You need to be really fit to do that," she says. "It's scary, and you need to go with really experienced people." (Related: What This Scary Scuba Diving Incident Taught Me About Proper Planning)

You might think of scuba diving as a leisurely activity, but it's actually considered an extreme workout. In just 30 minutes, it can burn up to 400 calories for the average woman. And considering most diving excursions last longer than 30 minutes, it's not uncommon to burn 500+ calories with just one scuba session. (Too scared to get in the water? You can rock scuba-inspired fitness gear without getting wet.)

Even though scuba diving was a surprising experience for Holmes, it was definitely worth the hard work and effort. "I did it in Cancun and then again in the Maldives," she says, adding that she's seen coral, sea turtles, stingrays, and lobsters on her excursions. "I've learned how to practice staying calm, staying present, and being grateful."