I often note that every marathon is full of unexpected surprises. No matter how many you have done previously, you can’t expect your next marathon to unfold predictably. The distance is too great, the number of variables too large. You never know what awaits around the next corner, or in the next mile. It could be wonderful—a loud, rhythmic rock band, or a cheering fan in a delightful costume. It could be tragic.

The 2013 Boston Marathon held more uncertainty than any race I have ever run. I began in a completely cheerful mood on a gorgeous day. In fact, I felt quite special to be running Boston again on the forty-fifth anniversary of my win in 1968. I was the oldest returning champion in the field. I confess to swelling with pride.

I also had great company on the road in the persons of John and Megan Valentine. John, a cancer doc from Vermont, was my oldest training partner. We had first run together in the early 1960s. Fifty years later, still fit but considerably slower, we still enjoyed each other’s company. His daughter Megan was young, strong, and fast, not to mention a motormouth. She was our cheerleader. She kept up a steady stream of encouragement.

While we ran the course, my wife, brother, and kids drove the parallel roads, stopping every 5 miles or so. They carried large, colorful “Go Amby” signs to the course-side, and greeted me with hugs and kisses. My sister, a below-the-knee amputee since childhood, was sitting in the finish-line stands, waiting to applaud my final strides.

By now I suspect you are remembering something about the 2013 Boston Marathon. Wasn’t that the one with the bomb explosions at the end? Didn’t several people die? Yes, and yes. But until the first bomb detonated at 2:49 p.m. Boston time, none of us had any reason to expect a tragedy.

Indeed, John, Megan, and I were enjoying a perfect run. We were hitting our planned pace, designed to get us to the finish in a time of 4:15. My brother, Gary, drove the family van through the congested Boston streets without incident. I saw everyone at all the appointed spots and felt elevated by their support.

The more miles we put behind us, the better we felt. Framingham. Natick. The screaming girls of Wellesley College at the halfway point. Our marathon couldn’t be going any better. We topped Heartbreak Hill in complete control at 21 miles, and soon started the long run down Beacon Street to Fenway Park.

There we passed the 25-mile marker. We knew every inch of the remaining course. First Kenmore Square. Then a turn onto Commonwealth Avenue before the famous “right on Hereford, left on Boylston.” I started to get giddy. Who else in the race could say they had first run Boston forty-eight years earlier, and won the whole thing forty-five years ago?

Nothing could stop us now, not with only a mile remaining. At least nothing we had ever experienced before.

Suddenly I noticed a knot of runners in the road ahead. They didn’t seem to be moving. Strange. When we got there a minute later, the knot had grown into a crowd. I could see a few police ahead, but no one knew why they had stopped us.

My cell phone rang. I had carried it this day for the first and only time in my marathon-racing life. I figured I might need it to stay in touch with the family van.

“Hello,” I said.

“It’s Cristina.” My wife. We are incapable of chatting without jokes and wisecracks.

“Hey, you’ll never guess—” I began, planning to tell her about the weird blockade.

“Shut up,” she said. Words she had never directed to me before, nor since.

I had never heard such urgency in Cristina’s voice, so I stopped talking and listened.

“We’re getting reports of a bomb explosion at the finish,” she said. “Don’t you dare keep running. Get back to the hotel. I’ll meet you there.”

Now I was shaking with a fear I didn’t fully understand. Even though we were on Commonwealth Avenue only a mile from the finish, we had not heard or seen any bombs. Several streets of brownstones and other taller commercial buildings blocked everything.

On our eight-block walk to the Sheraton Hotel, we saw dozens of sirens-blazing police, fire, and medical vehicles. The response was amazing. We watched in mute shock. There was nothing we could do.

Over the next hours and days, we absorbed the full horror of the situation—the two explosions, the deaths, the maimed, the nonstop manhunt, the great city shaking with terror and then struggling to right itself. We heard President Obama give one of his finest speeches, including the words, “We carry on. We race. We strive. We build and we work and we love and we raise our kids to do the same. And this time next year on the third Monday in April, the world will return to this great American city to run harder than ever and to cheer louder for the 118th Boston Marathon. Bet on it.”

I came back the next year with John, Megan, and many other family and friends. We had unfinished business to take care of. In fact, I have returned to run Boston every year since 2013.

In each I have followed three new personal traditions. First, I run for the Martin Richard Foundation, wearing its shirt and honoring Martin. He was the eight-year-old killed next to the finish line in 2013. Several months earlier, after the shootings at the Sandy Hook Elementary School in Newtown, Connecticut, he had painted a squiggly poster that pleaded, “No more hurting people. Peace.”

I now run with a small business card that I hand out mostly to youngsters lining the course. They represent the next generation of Boston runners and spectators. My card reads, “Thank you, Boston Marathon fans. Your cheers and constant support are what make Boston the world’s greatest marathon. Signed, Amby Burfoot. First Boston, 1965. Boston winner, 1968.”

Lastly, when I reach the finish line, I do not run triumphantly across it. Once I did. I sprinted across the finish as fast as my tired legs could carry me. Now that seems all wrong. Winning is out of the question, my time insignificant.

Now I am in no rush. I have nothing to prove. To arrive here, I have already covered 26 miles. So I stop to walk. I want to appreciate these precious moments. I want to celebrate life—mine and all others.

I stop at the point where the second bomb exploded in 2013. I slow to a walk, and glance skyward in silent prayer and thanksgiving, though I am not a conventionally religious person. I turn backward to applaud the runners streaming toward me.

I turn forward again, mere yards from the finish line, and keep walking. I note the spot where Martin Richard and family stood in 2013, so very, very close to the finish line. Life is so precious, and so fleeting. If only I could hold on to this moment forever—this reaching the glorious Boston Marathon finish line.

I know I can’t. But I can walk across the finish slowly, appreciatively, with gratitude. I can retain the memory in my mind’s eye. It will sustain me for another year, and perhaps longer.