Lessons From Tony Horton's New Book 'The Big Picture'

These days, Tony Horton seems like a fitness freak -- a genetic anomaly who can do more with his body at 55 than many of us can in our teens and 20s.

But before he created the popular P90X workouts -- before he mastered moves like donkey kicks and superman crunches -- he recalls that he excelled only at "excelling at nothing." "I was somebody who struggled," he says. "I was in trouble. I was broke and out of shape and in a bad way, and I was often depressed, overwhelmed and underproductive."

In his new book "The Big Picture: 11 Laws That Will Change Your Life," out today, Horton describes the principals that he credits with his success. Though all are connected in some way to health and fitness, they also have larger life applications, he says -- lessons he hopes his readers are eager to adopt themselves.

"I didn't have great mentors or teachers or coaches growing up," he says. "A lot of this was self-taught, and my sources were books." He pauses and ticks off titles on his bookshelf: Scott Peck's "The Road Less Traveled." Gary Zukav's "Seat of the Soul." John Ratey's "Spark," and Chris Crowley's "Younger Next Year." "Those were the early days of the 'Tony Horton, get your act together' story," he says. "And now I've condensed the things I've been working on for 30 years into one book."

Among the life laws Horton outlines in "The Big Picture:"

Do your best and forget the rest. This is the mantra that plays on constant loop in the back of Horton's mind. Show up, do your thing and do it well -- that's all you can (and should) worry about. When Horton got his first on-camera trainer gig in 1996, he was paralyzed with fear that he would blow it. He couldn't eat or sleep, and he contemplated backing out. The only reason he didn't? Those guiding words to do his best -- and you know the rest. Equally important, he says, is keeping in mind that "expectations and reality don't always coincide." Your best today might not align with your best last Thursday, and that's OK. Maybe you're under the weather or stressed out today, and you weren't last week. "Not every day is going to be your best day," Horton says. "Kobe Bryant sometimes scores 60, and sometimes he barely scores 10. That's just the nature of life."

[Read: Tony Horton Talks P90X and What's Next.]

Find your purpose. Horton knows what his is: He's dedicated his life to helping others find their purpose. But that's likely not yours. When it comes to exercising and eating right, he says, consider: What can you do to make today better? What can you do today to be happier, to be a better parent and to release serotonin so your brain functions better and you're enthusiastic and happy? And there you have it: your purpose. "We care too much about our appearance and what other people think," he says. "And we care too much about the numbers on the scale, the tape measure and dress sizes. These things have never inspired or motivated anybody. They've just created frustration." Instead, he says, latch onto something "more authentic and inspiring."

Embrace variety. If you want to be less stuck in life, Horton says, you need to be open-minded. He considers spontaneity and creativity the cornerstones of variety, and stresses the importance of working on your weaknesses as much as your strengths. Runners often want to stick to running; body builders to weightlifting. But "if you're trying to avoid boredom and injuries, and you want to continue to see results, do the things that aren't necessarily in your wheelhouse," Horton says. And that extends beyond fitness: "Maybe you've never had a working passport," he says, talking rapidly in excitement. "France is unbelievable, Italy is insane, Japan is super cool, South America is awesome, you know? There's so much out there. I'm not telling you to do back flips on your skis -- but do scary things that don't kill you." And as for Horton, who preaches that it's never too late to do what you want to do? He breaks into song mid-interview and declares: "Either in a future life or this one, I want to be a blues guitarist and singer."

[Read: Tony Horton: Why I'm Excited About My New P90X3 Workout.]

...and intensity. You're only doing your best when you continue to progress. And you only progress by adding intensity. So reasons Horton, who firmly believes in the power of pushing oneself. He recalls a time when he showed up at the gym and phoned it in: "eight of these, 10 of those, 15 of that other thing." He only ever tried 10 pullups -- until he realized: He ought to be fighting for one more rep. Today, in his 50s, he can do 40 consecutive pullups, and he credits adding intensity to his workouts with helping him build the strength he needs to continue working out hard at his age. "If you don't focus on getting a little bit better at things, then eventually even people who exercise on a regular basis will see their range of motion decrease and their endurance decrease and their strength and muscle tone decrease," Horton says. "They won't get the benefits of regular exercise like somebody who's focusing on getting a little better every day." Still, he cautions that intensity doesn't mean sacrificing form and function, which should remain top priority.

[Read: 7 Mind-Blowing Benefits of Exercise.]

Love it or leave it. Horton loves skiing. He loves the speed and the challenge and the way it pushes him physically, mentally and emotionally. He's not so big on bench presses and bicep curls and lap pulls. Too often, we cling onto things that no longer serve us, he says: "Here you are doing this thing, and you've been doing it forever. You've been going to the same class, you have the same teacher and you've been doing the same routine. And at one point, maybe you did love it. But you have to recognize when you no longer do." A clear test: If you don't look forward to it anymore, you probably shouldn't be doing it. And why's it so important that we love what we're doing? The short answer is that it means we're having fun, Horton says -- and life is "too short not to explore things you love."