The conditions for this year’s Boston Marathon were famously brutal, claiming many victims among the race’s 27,000 participants. Among them was professional runner Kellyn Taylor, who dropped out at 20K with symptoms of hypothermia. In a tweet posted later that day, Kellyn wrote, “I wonder if I just wasn’t tough enough to weather the storm.”

I got to know Kellyn pretty well during the 13 weeks I spent training with her Northern Arizona Elite team last year, and based on this exposure I can assure you that her blunt self-criticism right was right in character. Toward the end of my stint in Flagstaff, Kellyn, who is training to become a firefighter, tweeted out the news that she had “failed miserably” in a standard firefighter physical fitness test, which requires participants to complete a series of tasks in three minutes or less. When I discussed Kellyn’s “miserable failure” with her during an easy run a couple of days later, I learned that she had missed the cutoff by just 12 seconds!

As you can see from these two examples, Kellyn Taylor is highly self-critical, but in my experience she is not unusually self-critical for a champion athlete. Indeed, self-criticism is part and parcel of the champion’s mindset—an essential part of the mental formula for success.

This is not to say that all self-criticism is good. As a form of self-talk, self-criticism can be symptomatic of two very different things: high personal standards and low self-esteem. I believe that too many athletes and coaches view all self-criticism as problematic and fail to properly distinguish low self-esteem and high personal standards.

Low self-esteem is a consequence of caring too much about what other people think—or what we think other people think. When we compare ourselves to those around us and decide we don’t measure up in important ways, we tend to develop a generalized sense of low self-worth that can hold us back in life in a myriad of ways.

I have a runner friend who struggles with low self-esteem. As much as she loves running, for a long time she refrained from investing herself more deeply in her pursuit of improvement because she felt that she somehow didn’t deserve it. Only when she fell in love with a guy who helped build her self-esteem did she break out of this pattern. With her boyfriend’s support, she cleaned up her diet, started foam rolling, and began to do various other little things that she hadn’t done previously because she felt she wasn’t good enough to bother, and her running took off.

But this isn’t an article about self-esteem. It’s an article about the far more overlooked matter of personal standards of character. In my view, there is no better way to feel good about yourself and to have a positive influence on other people than to hold yourself to high standards of character, and endurance sports offer a terrific forum for character development.

What do I mean by character? A grab bag of qualities including discipline, positivity, steadfastness, and courage that contribute to success in life. However much or little you possess of these qualities, their limits will be tested in the context of endurance training and racing, and it is precisely by testing the limits of our character that we strengthen it.

It doesn’t happen automatically, however. What is guaranteed is that endurance training and racing will expose our lack of discipline, positivity, steadfastness, courage, etc. What is not guaranteed is that we will admit these lacks and set about addressing them. This is where self-criticism comes in. If we’re not willing to admit to ourselves the character flaws that hold us back as athletes, these flaws will continue to hold us back.

Ironically, low self-esteem itself is an impediment to healthy self-criticism based on high personal standards of character. That’s because it takes a certain degree of confidence to tune out society’s judgments and be your own judge, grading yourself in areas that do matter (e.g., how steadfast you are) instead of things that don’t matter (e.g., how you look in a swimsuit). So, if you currently lack self-esteem, you may need to work on that before you turn your focus to character development.

In these matters I speak from personal experience. In my forthcoming memoir, Life Is a Marathon, I recount “the day I discovered I was a coward,” which was the day I intentionally missed the start of a 3200-meter track race during my junior year of high school because I feared the pain. I’m sure some people will read this and think I’m being too hard on myself. But I’m glad I called myself a coward, because calling myself a coward was the thing that spurred me to work on gaining courage, and consciously working on gaining courage was the thing that transformed me into the ballsy athlete I am today.

In summary, self-criticism grounded in high personal standards of character is an effective tool for improvement. The proof is everywhere. Let’s go back to Kellyn Taylor. In her next marathon after Boston, Kellyn claimed victory over a strong field and recorded a time (2:24:28) that only six other Americans have ever exceeded. And the next time she took the firefighter physical fitness test, she passed.