OF late, a group of what may kindly be called wrinklies snakes past my window on the world several times a week.

Wearing boots, violently coloured anoraks - so they can easily be spotted in the dark by rescue helicopters - and hats made of yak-skin, they could be members of an expedition destined for the Antarctic.

They move slowly, deliberately, as if through porridge, each of them carrying a hiking pole in both hands, even though the only climbing they are likely to do is from kerb to pavement.

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On their backs, they have rucksacks which doubtless contain bars of Kendal mint cake (ideal for boosting energy levels), a flask of Bovril (to ward off hypothermia) and the wherewithal to call for help should it be needed.

According to a neighbour, they are retirees who're determined to keep fit and active while enjoying the great outdoors. They look well on it; some of them even appear to be enjoying themselves.

Nor will you find me among their mockers. On the contrary, I applaud them. When all around are piling on the pounds, it's good that there are still some people who are determined to keep in shape.

There's bound, of course, to be physical benefits to such exercise. As everyone must appreciate by now, a few minutes of movement, be it just your fingers and toes, and you can count on living who knows how much longer.

But what is less obvious is the effect on our mental health. No more, thanks to a study conducted by an American psychologist, which suggests that by taking a brisk walk three or four times a week we may be able to "grow back" our brains, thus warding off neurodegeneration and reducing the possibility of contracting Alzheimer's.

Kirk Erickson found that, when he got 60 people aged between 60 and 80 to walk briskly for 30-45 minutes three to four times a week the hippocampus region of the brain, crucial for memory, grew in size by around 2%.

Hence, Mr Erickson's assertion that "the brain and cognitive function of older individuals remain highly plastic. It's not this inevitable decline that we thought it was." In short, it's up to us if we want to avoid going demented. I do hope that's the case. By necessity, I have always been a long-distance walker. For this I have my father to thank.

He was not that much of a walker himself, regarding the distance between our front door and the car as "a stroll". He was of the generation which regarded walking as a waste of time and which cars had been designed to make redundant.

Naturally, he was keen that I learn to drive. But it was not a gene I possessed. There were several factors in my disfavour. First, and probably foremost, was my inability to distinguish between left and right, which led to some heated exchanges when I was given my sole lesson in the local swing park.

No less of a problem, though, was my lack of concentration. When you drive you need to keep your wits about you. Or so I have been told. "You've got to have eyes in the back of your head," was my father's advice, ignoring the fact that he'd neglected to put any there. When I told him I'd rather read than drive he said I could do one or the other but not both.

It was then that I took up walking seriously. Among my early heroes were the likes of Eric Newby, Patrick Leigh Fermor, Laurie Lee and Dervla Murphy, all of whom were never happier than when walking in the Hindu Kush or across Europe.

I wanted to do that, too, but the longest walk I ever undertook was the Southern Upland Way. Arriving at Tibbie Shiels Inn at St Mary's Loch after covering 15 miles in broiling heat, the dominatrix behind the bar struck a sympathetic note. "Nobody asked you to do it," she said.

Now I walk mainly in cities. Parisians call this flaneuring which may be defined as strolling without purpose. This is perfect for someone with my sense of orientation.

I go where my inclination takes me, stopping frequently to refuel or window shop or eavesdrop on conversations. And all the while I've been growing back my brain. Who'd have thought it!