They gave me a certificate and a t-shirt, and some very useful parting words of advice. After 36 early-morning sessions, spread out over three months, I finally had graduated from cardiac rehab.

Each of the 36 mornings, beginning two months after my near-fatal heart attack, I got weighed, had my blood pressure and pulse taken, attached color-coded electrodes to my body, and then spent 50 minutes or so on the treadmill or a stationary bike.

It was reassuring, so soon after almost dying, to have physiologists and cardiologists and other staff watching over me, checking the data from the electrodes, noting my target heart rate, asking me how hard I was working, making sure that I was pushing myself but not pushing myself too hard.

I knew that the foremost risk factor for having a heart attack is having had a heart attack. The cardiac rehab staff was there to make sure that didn’t happen, at least not on their watch.

I was tentative when I started rehab. My heart attack had come with absolutely no warning, just a couple of days after I had run a couple of miles. Couldn’t it happen again if I started walking fast or even jogging?

But little by little, session after session, I became more comfortable exerting myself, more confident that I could do this, more certain that there was no permanent damage limiting what I could do.

On my last morning, right before I received my graduation certificate, one of the physiologists congratulated me on how well I had done, how much progress I had made. And she cautioned me that not every day would be so good, that the trajectory of my recovery would not necessarily be linear.

There will be days, she said, when you can’t do better than you did the time before. It happens, she said, even to people her age—27. We all have bad days or days that just don’t work. We can’t always go faster, can’t go further, can’t swim that extra lap or lift that extra weight. Not at her age and definitely not at mine. Don’t be too hard on yourself, she told me.

The next morning, on my own, no physiologists in sight, I went out and ran. I ran slowly, and not terribly far. In my ear, I could hear the physiologist’s voice, I could sense her presence. I was reassured.