I started last summer, and it was incredibly hard. It required a total reset in how I thought about myself and what I can do.

I had always thought that, at some point in life, most people become “who we are.” Our lives are built around whatever that is, and no matter what we might actually be capable of, this idea keeps us fixed in one place.

At 35, I thought I was “who I was.” I didn’t think it was still possible to improve significantly in anything, let alone something involving my body. Our culture is fixated on youth, on potential, on lists of “30 under 30” — especially for women, who are assigned a “biological clock,” whether they believe in it or not.

I had to dismantle all that. To qualify, I had to run a marathon pace of 6 minutes 17 seconds per mile. For me, that’s usually a sprint. When I started training on a path along the Hudson River, I couldn’t even hit that pace for one mile. I tried anyway, over and over. I would stagger home afterward, late at night or before work or both. Once, at the end of a workout, I cried out, “Oh, God!” so loudly that a man across the path looked up at me in alarm. But I kept trying, repeating something someone told me about “getting comfortable with the uncomfortable.” It never got comfortable.

But it did become possible. One evening in the fall, after logging 22 miles in Central Park close to my goal pace, I paused. Who did I think I was?

That was the point. After months of work, my physical and psychological reframing had worked. It wasn’t an idea anymore. It was my plan. We don’t have many opportunities later in life to change who we are, without worrying about what other people think or upending wherever we’ve landed in our lives. We especially don’t have a lot of ways to do that physically. I had internalized a narrative about my body — that once I turned 30, there might not be much to look forward to. I didn’t know the opposite could be true.