The Sagamore Bridge crosses the Cape Cod Canal, which cuts major mileage from the voyage from Buzzards Bay to Cape Cod Bay and Boston. Unsurprisingly, the early English settlers of Massachusetts, who had to wrestle 17th-century sailing craft around the cape through foul weather and winds and past shifting shoals, first dreamed of it. There were two tidal rivers reaching toward each other from opposite sides of this narrow neck, with a portage between them. Everybody from Myles Standish to George Washington took a crack at designing a canal, but the current version of it wasn’t finished till 1916. The late stages were hurried along, no doubt, by the presence of German U-boats waiting offshore to pick off ships forced to run the long gauntlet from Falmouth to Provincetown. It’s unlikely that very many of the tens of thousands of folks who will pour onto the Cape today will reflect on the history of this traffic bottleneck.

The crew and I have had a fascinating two days here on this classic example of glacial geology. We’ve been here not so much to dwell on the natural history of the place as to walk, talk, and paddle with a man who’s experiencing – and recording – a dramatic personal history.

Greg O’Brien is 65. A longtime journalist and editor, he’s slim, energetic, athletic. But he has a family history of Alzheimer’s Disease, and watched both his maternal grandfather and mother slide into the shadows of confusion, loss of self, and death. He also has a predisposition – a nasty gene usually called APoE4 – and at the age of 59, after a severe head injury suffered in a bicycling accident, began exhibiting discernible symptoms. His doctor opined that the injury had “unleashed a monster.” But he’s refusing to go gentle into the night. He’s written a brilliant book about his battles with the ultimately invincible monster, titled On Pluto: Inside the Mind of Alzheimer’s.

Early symptoms generally trigger denials. But after his diagnosis – a blunt, frank assessment that softened no blows – Greg began, delicately at first, to break the news to his kids.

Once euphemistically termed “hardening of the arteries,” the disease was first described clinically as “presenile dementia” by Bavarian-born psychiatrist Alois Alzheimer. When a young patient of his died after showing behavioral symptoms and loss of short-term memory, he examined the man’s brain and found “amyloid plaques and neurofibrillary tangles.” Soon afterward, the condition assumed Dr. Alzheimer’s name. It will be epidemic among the Baby Boomers. At the present rate of increase, the number of sufferers is projected to triple during the next generation.

The reason our crew has been here is that Greg’s neurologist recommends vigorous regular exercise, both physical and mental, and Greg has combined that prescription with his love of the outdoors and the abundant natural possibilities of the Cape. He isn’t allowed to hike alone anymore because of occasional confusion; so he either hammers the treadmill at the gym for a few miles or walks the beaches and woods with his son Conor. The two of them also kayak in the tidal lagoons and bays inside Nauset Beach, on what’s called the Lower Cape.

We took a beach walk yesterday. It was old memories for me; I worked on that bay as a pearl diver in a summer camp in 1951, and this was my first trip back. We talked for an hour, with the camera rolling, in his office, a small shingled building across the yard from his house. Its walls are hung with memories – Kennedy’s assassination, Nixon’s resignation, and various journalistic awards. Greg still writes, though he says it takes longer now, but he talks up a storm. Every few moments he pulls out his iPhone and types into it, preserving the short-term facts the rest of us take for granted we’ll retain. Conor, who’s become his designated caregiver, sticks close by.

This morning we gathered at a marina near Eastham, rented kayaks, and paddled vigorously up a bay on the slack tide, the camera crew following in a canoe. Shingled and clapboarded mansions – cottages, I think they call them – lined one shore. A pair of gulls feasted on crabs on a gravel bar; an osprey, hunting a meal for two very large chicks perched at the edge of their nest and exercising their still-new wings, swooped and dove for fish ahead of us; beside us, a snowy egret stalked thoughtfully through the shallows, cocking his head atop an unbelievably skinny neck; and far up, an eagle soared, grandly superior to the whole scene.

Greg had a destination in mind that I thought a bit ambitious, especially when it turned out to be downwind (you’ve always got to get back). But we did it just fine, took a few videos of us paddling through salt marsh, and finally headed back. I find kayaking a bit of a pain because of a frequent need to change positions, very difficult while entombed in a narrow hull. But I reasoned that since it hurts the same whatever speed I go, it will hurt for a shorter time if I go fast; so I did.

We parted at the marina. I couldn’t resist a farewell I got from his book. “Greg,” I said, “don’t forget that hundred you owe me.” He grinned; he still hasn’t lost his sense of humor.