The night before we are due to climb the Old Man of Coniston, Sion Jair calls to warn that, chances are, we won’t be able to go up the 803m Lake District fell in the morning. I look out the window of my hotel. There’s little wind, the temperature is mild. I don’t say anything, but he seems to read my mind. “It’s quiet down here,” he explains, “but at the top, everything is exaggerated. The wind will be 10 times as strong.”

Jair is certainly no fair-weather climber. Now 68, he first came to the Lake District in 1968, half a century ago. He’s scaled the Old Man of Coniston, he guesses, on more than 5,000 occasions. He often goes up twice in a day; in winter, he just sticks on crampons. For many years, he has offered courses on mountain navigation and, informally, he has rescued dozens of walkers who have underestimated this craggy Lakeland icon. As much as anyone, Jair is the old man of Coniston.

More remarkable is that Jair has continued this regime despite a succession of debilitating illnesses. For years, he suffered from chronic fatigue syndrome. His doctor suggested a change of air might help him, and in the early 2000s he swapped Spaghetti Junction in Birmingham for Ulverston. The new locale suited him, but then he was diagnosed with pernicious anaemia, a condition in which the immune system attacks healthy cells.

Pernicious anaemia can be moderated with injections of vitamin B12, but Jair’s body rejected the treatment. He says: “The doctor and a specialist told me, ‘If you don’t have the injections, the most you can expect is to live for three years. And it won’t be pleasant.’” Jair, though, kept walking. Initially he would be exhausted from even short hikes but, over time, his tolerance increased drastically.

“My body had adjusted to the small amount of B12 I could accept,” Jair goes on. “The doctors did tests and put it down to – although it’s not scientifically proven – my exercise routine. I just kept going and my body had two choices: I could either sit down and die, or the body had to get up and use what it had.”

But Jair’s medical issues didn’t end there. The physical effects of the pernicious anaemia had concealed a severe mental deterioration. “I had a brain scan – and they found one,” he says, wryly. “But my brain had shrunk. When they told me I had Alzheimer’s it was a relief in one way, but then they said: ‘There’s nothing we can do to help you because it’s too far gone.’ It had gone past the early stages.”

This was about four years ago, and Jair’s response was exactly the same as he’d given to previous ailments: he laced his boots and went walking.

Sion Jair: ‘Coniston Old Man has got absolutely everything for everybody: steep climbs, gradual climbs, beauty spots, atmospherics.’ Photograph: Murdo Macleod/The Guardian

The following morning, I meet Jair outside a pub at the foot of the Old Man of Coniston. He has the jolly, ruddy complexion of a life spent outdoors and looks a full decade younger than he is. Most days, he reckons he can be up and down the mountain in a shade over two hours, which is very fast and pretty well the same pace he’s always done it. Today, he looks warily at the clouds and decides we might just be able to fit in a quick summit, so long as we don’t spend too long admiring the views. He sets off at a clip and I scamper behind him.

There is no reliable cure for Alzheimer’s disease and it is notoriously hard to slow down. Still, Jair believes – and his doctors support him – that regular, vigorous exercise is as beneficial as anything. In this, recent research appears to back them up. Most studies to date have focused on whether exercise might delay the onset of Alzheimer’s, but a 2017 paper from the University of Kansas looked at 68 men and women who had already been diagnosed with the condition. For six months, half the sample increased their programme until they were doing 150 minutes of brisk aerobic exercise a week. The other half did stretching and light toning. At the end of the test period, almost all had shown distinct improvements on tests for everyday physical skills. Also, some of the walkers significantly increased their scores on cognitive tests that focused on thinking and remembering. The brain’s hippocampus, the area most closely linked to memory retrieval, had in some cases actually grown.

It’s a small sample, but Jair isn’t surprised. When asked why he walks – and indeed why he climbs the same mountain day after day – he offers two answers. First, the longer, more romantic one. “I’ve been up every single mountain in this country, quite a few of them more than once,” he says. “Coniston Old Man has got absolutely everything for everybody: steep climbs, gradual climbs, beauty spots, atmospherics. Very underrated mountain really. You get fantastic views and all sorts of phenomena: broken spectres, fogbows, which are very unusual, times when you’re looking over the clouds. I’ll have done all that, seen all that beauty, had my exercise and people are just getting out of bed.”

Jair smiles, and then offers a shorter, alternative version. “And it’s stability,” he says. “Something I can do.”

Jair has heard the advice that you should test yourself mentally, do sudoku, crosswords and the like. But this doesn’t work for him. Not long ago, he tried to do a “coffee time” crossword. He did it every day, in five minutes. But now he just stared blankly at the newspaper. “Do you know, I couldn’t do two or three clues,” he says, shaking his head. “So all that did was discourage me.”

When he reads, Jair finds that he has forgotten the start of a sentence by the time he’s finished it. Nevertheless, as we wind our way up the Old Man, he has a precise recall of events that took place 50 years ago, even down to very specific details about the one-month Outward Bound course that first brought him to the Lake District. “I had quite a good memory in the past,” he says. “But that’s ironic, isn’t it? I can remember that my memory was good, but I can’t remember what about.”

There’s a tinge of melancholy as Jair outlines the defining moments of his life. He came from a broken home, before becoming an engineer for Alfred Herbert, the dominant British manufacturer of machine tools for most of the 20th century. For many years, he was a Jehovah’s Witness. Then, in the early 1980s, he was made redundant and that started a slide from which he struggled to recover. Jair sums it up: “So I lost my wife, my son, my job and then my home all within 12 to 18 months.”

Through it all, however, walking and climbing were always a release – and they still remain so. Eventually, we come out of the protection of the valley and we are buffeted by a spiteful wind; up ahead is the survey column that marks the peak of the Old Man. Is it still a thrill to reach the top? “No,” he replies, to my slight surprise. “I got over this business of going up a mountain for a challenge years ago. I just do it because I enjoy it. I do it because it’s familiar, and particularly when you’ve got Alzheimer’s, you need something that’s familiar.”

The future, when Jair thinks about it, can be a bit scary. He has no family, no companion (though he’d dearly like one); the one thing that’s always there for him is the Old Man of Coniston. “All I know is that I’m not getting any better, but I don’t think I’m getting that much worse that quickly,” he says as we have a slug of water before heading back down. Then he laughs – after all he’s made a habit of defying conventional medical wisdom: “Anybody else they’d know where it was going!”

How exercise improves your memory

The Department of Health recommends at least 150 minutes of moderately strenuous physical activity per week for everyone. This breaks down to 30 minutes of activity per day, for at least five days a week. The Alzheimer’s Society offers the same advice.

Walking, which is free and requires no equipment, is recommended for anyone in the early to middle stages of dementia. Other forms of good, low-impact physical activity are gardening, swimming and tai chi.

Research has found that stress and personal problems decrease the brain’s ability to learn and retain information. Lack of sleep, excessive intake of alcohol and a poor diet also weakens the flow of messages between brain cells.

A recent paper from Brigham Young University in Utah backs up the idea that exercise can help to build lasting memories in the brain. The experiment – conducted on healthy male mice – exposed the animals to stress and found that those that exercised outperformed a control group that were forced to be sedentary.