David Bellamy's lifelong addiction to walking began when he was a child, living in London during the Second World War, under the shadow of the German Luftwaffe, in what he calls 'doodlebug alley'. He and his friends used their bikes to get out of the city, and went on long walks in search of some fresh air and a little peace.

'We discovered this amazing thing called the British countryside,' he says, 'which was then, for the most part, looked after by fantastically good farmers and excellent gamekeepers. That experience made me what I am. I still walk whenever I can, although I'm often too busy for day-long walks now.'

In fact, it was one particular walk, through the Highlands and along the coast of Scotland, that convinced Bellamy to become a botanist. 'I walked with my wife-to-be from Spean Bridge, near Fort William, up to John O'Groats,' he says. 'We had a book on the British flora, and on the way we quite simply discovered plants - huge numbers of plants.'

Bellamy now spends much of his time abroad, visiting conservation sites around the world. But he always enjoys returning to the British countryside.

His favourite walk, he says, is one which he first tried as a boy, and has done dozens of times since. It's the Devil's Punch Bowl, near Hindhead in Surrey - a huge, natural amphitheatre, often glimpsed from a car window by travellers on the A3 from London to Portsmouth.

The area has dramatic views, a variety of woodland and open spaces, and a fine history, including England's only surviving wooden Saxon windows, in the church at Thursley.

'I remember standing at the head of the Punch Bowl on a winter's day, with everything covered in hoar frost,' says Bellamy. 'I was about to start walking, when a warm weather front suddenly arrived, and all the frost fell off the trees, tinkle tinkle tinkle, in a matter of seconds. It was the sort of experience you'd never see from a motor car.'

Bellamy's main concern now is encouraging people to take greater care of the world's most important and popular sites of natural beauty. He fears that in the future there may be stringent restrictions on access to places such as the Lake District: perhaps only a single entry-ticket will be given out to each person in their lifetime.

'All tourists have got to be environmentally friendly, and those who enjoy the outside world have got to be the most careful of all,' he says. 'Fortunately there's been a bit of a green renaissance in the past five years, with more people trying to put the world back together. The tourist industry has also started trying to do the right thing - it knows now that if it doesn't put its house in order, its product will pretty soon disappear.'

· David Bellamy, president of the YHA, will be speaking at The Outdoors Show, at the Birmingham NEC, 15-17 March