Whenever I have a day at home I take exercise, usually in the late afternoon. For many years it was a walk. Then I began to feel that was not enough, so I took up running. Almost immediately I misjudged my step off a grass verge, turned my ankle on the tarmac and had to be scraped up by a passing motorist (apparently injuries sustained within two months of starting a new sporting activity are among the most common). That was when I took up swimming, which I have now been doing for three years or so. It’s dull but generally beneficial and I’ve got into the habit of it.

Sometimes, though, if I’m feeling particularly frantic, it doesn’t do the trick. The physical exertion simply compounds the state of mental overload and something less strenuous – like a walk – seems preferable. I’m lucky that within a few hundred yards of the house runs the River Tay, along whose bank stands the remains of a magnificent beech avenue, once the carriageway to nearby Murthly Castle.

Yesterday evening I was in need of such a walk. It was still and the river was flowing quietly. It’s broad and deep here and a succession of famous salmon pools lure fishermen into the peat-brown water. Sometimes they’re concealed from the path by the steepness of the bank and you don’t know there’s anyone there till you hear the swish of the cast and see, through the trees below, a line fly out into the current. The largest British salmon ever caught on a rod and line was landed from these waters in 1922. It weighed 64lbs and in the formal photograph, lying at the feet of its tweed-clad nemesis, a certain Miss Ballantine, it looks like something from the deeps of the ocean, a gaping leviathan.

There’s always bird activity on the river, although less when the fishermen are out. Apart from the customary squadrons of mallard, sometimes a goosander heads upstream, powering low over the water, and once in a while one of the ospreys from Loch of the Lowes lumbers overhead looking for its supper. Very occasionally there’s an iridescent flash as a kingfisher skims diagonally from one bank to the other. The far side is thickly wooded and rises out of view. I imagine that I could be in the backwoods of New York State or Massachusetts with a war party of Mohicans running silently through the trees opposite.

Further along, the wood gives way to a long walled garden that reaches down to the water, and beyond it a sprawling ochre-coloured house. This is Eastwood, where Beatrix Potter came to stay as a young woman and wrote The Tale of Peter Rabbit, thereby summoning another memorable image, that of Mr McGregor with his rake chasing Peter out of the very vegetable patch one can see across the water today.

Then, a few hundred yards downstream of Dunkeld and Thomas Telford’s elegantly arched bridge, one comes across the pièce de resistance, the Birnam Oak. This venerable giant has stood here for half a millennium, its outstretched limbs now supported on sturdy timber crutches. The only survivor of Shakespeare’s Birnam Wood, it’s a living relic of a time when the greenwood still cloaked swathes of Scotland. At the time the bard was sharpening his quill it was probably just a sapling.

Yesterday, by the time I reached the oak, a calm had settled over me. My mind had stopped whirling and I felt I had been taken in and embraced by the ancient trees, the steadily flowing water on my right and the stubble field on my left, where fat black crows sat in the long ridges of straw left by the harvester. I had been re-connected, not only to myself, but to something greater, in a way that can only happen in nature. It made me think of the phrase ‘nature deficit disorder’ used by Tim Dee, author of Four Fields, whose event I chaired in Edinburgh ten days ago.

His is one of the most beautiful books I have read in a long time and he returns again and again to the importance of our relationship with our natural surroundings. Yet according to the World Health Organisation more than half of all people in the world now live in towns or cities. By 2030, that figure will rise to 60% and by 2050, 70%. If nature deficit disorder isn’t widely recognised today, it surely will be by then – a thought that lends even a short evening stroll along a riverbank a new and particular poignance.