Original SEAL P;T. Course, Houston

John "Jack" Walston spent almost a decade as a US Navy SEAL, and even when he left the military, he didn't leave behind military workouts. Instead, in 1997, he started Original SEAL PT Course, bringing the training techniques he'd learned in the army to the public—and the fitness enterprise is still thriving 15 years later.

Walston and his team specialize in military-style boot camp workouts, which they run in Memorial Park five mornings per week. The 90-minute sweat sessions start at 5 a.m., and rotate daily; some focus on building endurance, while others tone the upper body with moves such as push-ups and extra-long waves. The coaches host workouts in more relaxed styles, too—some, like the JackFit BaseCamp, cater to people who already work out rigorously, while P.E. in the Park consists of a casual, yelling-free hour of calisthenics, open to all levels.