Sometime around noon, I started to appreciate the math: The fact that I was riding my bicycle to my 20-year high-school reunion meant that I wasn't quite as young as I'd been picturing myself all these years. I was creeping up a hill, badly out of breath, but the origin of this revelation was neither my legs nor my lungs. It was the two spots on my buttocks that made me wonder if the bicycle's seat was made of salt and razor blades: saddle sores.

I was not sure exactly where the idea of riding 200 miles along the Mississippi River, from my current home in Minneapolis down to my childhood home of Winona, came from. Maybe it was some kind of midlife bid to escape the icy grip of domesticity--nearing 40, I had a wife, two kids and a house. Maybe deep down I still wanted to prove something to my old classmates. All I knew for sure was that once the idea grabbed hold of my psyche it wouldn't let go, and now, just a few hours into the trip here I was, out on the road, loaded down with gear and memories and doubt.

"Hola," I said to the man walking past me. I was not far south of Minneapolis, but I was already on a part of the river I had no idea existed.

"Mucho pescado," the man said, pointing over to where his friends were fishing. He was from Mexico. He had cowboy boots and a belly that pushed out his giant belt buckle. He looked at my loaded bike and sweat-drenched shirt.

"¿A donde vas," he asked.

I told him where I was going. "A Winona."

"Es muy bonita, Winona," he said, then paused. "Pero, hace un dia desde aqui"

It's very beautiful. But it's a day from here.

"Si," I said. "Es muy lejos." Yes, it's very far.

Winona was beautiful. That was what everyone said about it, and about the valley that held the town. And it's true. The road that passes through it and winds up the Mississippi River has long been considered one of the most scenic drives in the country. Likewise, that segment of the 3,000-mile Mississippi River Trail for bicycles is one...