The world started whispering to me when I was 9 or 10. Actually, it had been talking to me since the day I was born, but I didn’t hear it until I was in third or fourth grade. The voice was subdued and gentle. It spoke through friends. (Kids notice things and say things without the filter of niceties, after all.) I felt it for a moment — a quick piercing of my self-confidence — and then it was gone. I didn’t understand what was being said. There was only a visceral understanding I never could have articulated at the time that, somehow, I wasn’t quite good enough.

By the time I was a teenager, the voice had swelled to a grating and unrelenting clamor. No doubt you’ve heard it. The voice that says to be smaller, thinner, prettier, younger, brighter; to do more and worry less; to charm the masses and get his attention. It is emblazoned on magazine covers at the grocery store and broadcast across television networks and movie theaters. It is sprawled across storefronts and billboards advertising clothes and swimsuits and shoes and shampoo. (Shampoo!) It masquerades as helpful (Be the best you!) and empathetic (Achieve the you you’ve always wanted!) and nifty (Fix that shortcoming-du-jour with this one trick!). The voice bellows and shrieks so that you don’t even notice when it sidles up beside you and whispers in your ear: You are not good enough.

But the voice does more than perpetuate a billion-dollar industry built on insecurities. It chips away at the confidence we were born with as young girls, the pluck and mettle we were meant to nurture and cultivate as women. Even worse, as the voice becomes louder, we often lose the ability to distinguish between it and our own, the voice that cheered us on when we were little and said, The world is yours. Go get it.

It wasn’t until my 20s, when I started distance running, that I realized how much self-doubt and fault-finding had crept into my everyday internal narrative.

But running, I discovered, simplified the chaos. It filtered out the unnecessary and the untrue until only one voice remained: my own. In the silence of the miles, I heard my voice as it should be. It was a whisper by that time, timid and out of practice, but with every mile it grew stronger and bolder and more fearless.

And so did I.

In the middle of a run, when I am pushing my body to run fast, to run far, to drive up a hill, the voice that says to be smaller, thinner, prettier, younger, brighter, doesn’t just fade away: it becomes irrelevant. There is only the sound of my feet against the pavement, the rhythm of my breathing, the cadence of my arms and the gentle reminder that this is who I am. Human. Strong. Powerful. Capable.

I have run well over a dozen marathons, and not once during any of my races did I think about fitting into a pair of skinny jeans. During training, I ask my legs to run 14 miles a day, and they do. I ask them to run workouts, and they respond. I ask them to run 20 miles in the dead of summer, and they say, “Sure. Why not?”

Because we are not compartmentalized but systemic, our triumphs over fear and self-doubt spill over into other areas of our lives. The more we surprise ourselves in running (or cycling or hiking) — the more we confront and conquer — the more we want to achieve outside the sport, the better equipped we are to do it and the more we believe that we can.

From a young age, women are trained to spot flaws and to obsess over them. The narrative is ubiquitous, and few — if any — of us are immune to the message. But we can learn to recognize lies from truth and protect the fierce and valiant spirit we had as girls, that we need as women.

I never feel more feminine than I do when I am running: empowered, uninhibited, brave. The miles eliminate the noise and the nagging and return me to a sacred silence in which I can finally hear the true, spunky, passionate voice that cheered me on when I was young. It cheers me on still: The world is yours. Go get it.

Amy L. Marxkors is the author of two books on endurance sports and a frequent contributor to running blogs and magazines. She has run a mess of half marathons, marathons and 50Ks, even though she swore she would never be a distance runner. Yet here she is, and she couldn’t be more thrilled.