What was it that provoked a genial, easy-going man of 65 to such exertion? To find out, I joined him for a day's march, on which we set off from the market town of Tetbury and, after a roundabout, 11-mile hike, finished up in the Silk Wood, deep inside the Forestry Commission's magnificent arboretum at Westonbirt.
In earlier days Roy worked for Guinness (where he met his wife, Sue) and for Courage; but the spur that goaded him into the long march was his imminent retirement from his post as chairman of the Trust. Feeling that he ought to go out with a flourish, he conceived the notion of a major fundraising tour: further, he decided that he should walk not only round the reserves, but between them as well.
His first step was to build up stamina through a six-month, one-to-one course with Adrian Clift, a young fitness trainer in Stroud. This cost pounds 1,000, but was paid for by Nuclear Electric, Roy's principal sponsors. (One anonymous donor gave pounds 1,000 outright, one couple pounds 1 a mile.)
With his weight down by half a stone and his upper body strengthened, and launched by an enthusiastic letter from Prince Charles, Roy set out on 21 March, and since then has walked for 39 days out of a total of 65. On all but two he had company: his biggest following was 28-strong, his next largest a gaggle of 13 girl guides.
In all the 400 miles he had only one unpleasant encounter. Out with three friends north of Gloucester, he came to a gate laced shut with barbed wire. Because they were on an official footpath, they climbed over and carried on - only to meet "a big, red-faced fellow at 30,000ft and rising fast" who claimed to be the landowner". When they offered to show him the map, he blustered that he couldn't read it without his glasses, and they left him to seethe on his own.
No such aggravation marred the morning I spent in Roy's company. Soon we were passing Highgrove, Prince Charles's country home, and noting with approval the weed-flowers - dandelions, buttercups, daisies - in his organic pastures. One of the main pleasures of the walk, Roy said, had been to see wild flowers reappearing in numerous grass meadows.
The sight of a buzzard overhead, under harassment by two rooks, reminded him what a come-back the big hawks have made - another result of less aggressive farming. A gloomier spectacle was that of hedgerow elms which had grown to a height of 20ft but were starting to die, victims of Dutch elm disease.
On we went through the gentle landscape, past Chavenage, a fine house of Norman origins, and along a bridleway to Beverstone, whose castle has remained in ruins since it was blown up during the Civil War. Legend relates that a young man in the Parliamentarian household at Chavenage loved a girl at Royalist Beverstone, and used to cross these fields at night to visit her. Who could say that we were not treading the very path he used for his nocturnal assignations?
As we walked, Roy spoke fondly of his ultimate destination, Lower Woods, a square mile of ancient forest near Wotton-under-Edge, famous for its nightingales. The block was offered to the nation in lieu of death duties by the executors of the 10th Duke of Beaufort, who died in 1984 after having expressed the wish that it should go to the Wildlife Trust; now it is the Trust's largest reserve by far, the jewel in its crown.
There, on Thursday evening, Roy was welcomed by the novelist Joanna Trollope - herself no mean conservationist - and his achievement was celebrated with a barbecue and music, some made by humans, some by nightingales: a fitting end to a notable peregrination.
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