No pain, no gain.

You’ve heard it many times. But do you apply it to your own life?

Katie Ledecky, 17-year-old swimming champion who looks very much like the next Olympic superstar, does.

After smashing her own world record in the women’s 1500-meter freestyle over the weekend, Ledecky referred to what it takes to compete on such a high level.

“It was painful,” she said, “but it pays off in the end.”

Ledecky was merely echoing the words of legendary swim coach Doc Counsilman, who urged his athletes through the water with the mantra, “pain, hurt, agony.”

Pain is a four-letter word that creates champions.

Here’s what my friend and co-author Rob Gilbert says about pain and progress.

“No matter whether you experience hurt, pain, or agony during practice, when the workout is over and you go home, you go back to feeling how you did before practice.”

To put it another way, for 22 hours of the day, champions and also-rans feel the same way. It’s during those two hours of practice in which champions — and you — can make the choice to endure discomfort.

Gilbert loves the quote of former heavyweight boxing champion Muhammad Ali, who said, “”I hated every minute of training, but I said, ‘Don’t quit. Suffer now and live the rest of your life as a champion.'”

It’s vital to know that the hurt, pain, agony need not be physical. When you must wake up earlier than others, it’s painful. When you must choose between going out or training, it’s painful.

It’s painful, but it’s a choice you can make. No pain, no gain.

My friend and mental skills coach Dave Cross of “Yes, I Can!” first taught me this affirmation: I do the thing that is hard to do, and the power comes.

Ledecky has learned this while still in her teen years. It could bring her glory at the 2016 Summer Olympics in Rio de Janeiro. But more important, it gives her — and you — a blueprint for a life of achievement.

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Greatness begins with the way you think, and “Think Better, Win More!: How Sport Psychology Can Make you a Champion” will help.

To improve at anything, check out “The Improvement Factor: How Champions Turn Practice into Success.”

Need some motivation? Open any page of “Thank God You’re Lazy! The Instant Cure for What’s Holding You Back” for a story or a quote to lift your spirits.

Are you a coach? Here’s “Ten Things Great Coaches Know” by Mike Tully and Gary Pritchard.