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The year Teri Smith turned 40, she decided it was time to tackle a nagging urge to get in shape. That's when she signed up for her first fitness boot camp class.

"I didn't have any energy and I could feel my age creeping up on me," says the Pembroke Pines., Fla., mother of two. Her weight had crept up, too. Though she wasn't heavy, the 20 pounds she'd added over the years didn't feel comfortable on her 5-foot, 3-inch frame.

A successful graphic designer, Smith had never felt as comfortable in sneakers as she did in front of her Macintosh. "I didn't have the confidence" to exercise, she says, because she always felt uncoordinated.

But when her stylist suggested she try the boot camp fitness class, she worked up the nerve to do it. And she's never looked back.

"No other workout makes me feel so good. It makes me feel like I really kicked my butt," says Smith, 42, a faithful boot camper for 2 1/2 years now.

She is now one of the fastest runners in the class, and is once again happy with the way she looks. "It took 12 weeks, but I lost 25 pounds," says Smith.

What Is a Fitness Boot Camp?

Boot camp exercise classes vary in style, depending on the teacher. But you can generally expect to meet outside, rain or shine. You'll probably spend an hour doing some form of cardiovascular exercise (running, hiking, interval training, or obstacle course challenges), along with strength elements (using dumbbells, exercise bands, or the resistance of your own body weight). You'll also work on flexibility in a stretch portion of the class, which may incorporate elements of yoga or Pilates.

The fitness boot camp class Smith attends in Weston, Fla., is taught by ex-college football player and fitness coach Tom Rayhill. Rayhill's boot camp is offered three times a day, seven days a week, year round. People can pay daily, weekly, or monthly and come as often as they like.

Many other boot camp classes are offered for defined periods of time. John Spencer Ellis' California-based Orange County Adventure Boot Camp, which has locations in nine countries, is offered to women only at 5:30 a.m., five days a week for one month. Many participants re-enlist one or more times.

Boot camp fitness classes challenge the mind as well as the body, instructors say.

"We work on technique, form, core training, breathing, relaxation, and a better understanding of how the body moves," says Ellis. "In four weeks, people will drop 5% to 6% body fat, lose up to 10 pounds, and reduce their mile time sometimes by 2 minutes a mile."

Often, the confidence participants gain in boot camp class helps them take control of other aspects of their life, Ellis says.

For more info: adventure boot camp class