"It's very simple. If you don't push yourself nobody else is going to do it for you."
Long-time Smith fans reading the new book may be surprised at the extent to which the action in the 85-year-old's fiction is derived from his own experiences.
"The only thing more pleasurable than having a remarkable adventure is living to tell the tale in new and unexpected ways," says Smith. "Throughout my more than 50 years of writing novels I established a well-worn pattern: experience life to the fullest through my research and live through what the characters in my novel live through - hiking while experiencing severe dehydration up the Rift Valley in Kenya, paddling down the Nile with the ancient Egyptians, avoiding a shark attack while scuba diving in the Seychelles - and then release the dogs of my imagination when I am back at the comfort of my writing desk in Cape Town."
Which also explains why he has only now written a memoir.
"I always thought my novels were my life story. But over the past 10 years, more and more of my fans wanted to know more about these adventures and the know-how they created about the worlds I've inhabited.
"It felt that now was a good time to pass on these experiences to my fans by telling my own story."
On Leopard Rock doesn't follow the all-too familiar memoir pattern of I-am-born-go-to-school-discover-girls-struggle-to-build-a-career-find-success.
Instead it's divided into chapters on themes such as "This Seafaring Life", "This Hunting Life", "This Diving Life" and so on, each focusing on one of Smith's enthusiasms, of which there are many, which has meant a constant supply of experiences on which to draw.
"I write from a deep desire to tell stories I think are important," he says, "and to link those subjects which are very close to my heart. Because they're close to my heart I can write them with all my heart. Mine comes from my upbringing in Africa, and my fascination with certain aspects of African life.
"You have to believe in your own story, and then to have the enthusiasm for it, if you believe in it, then you have the enthusiasm, but if you're faking it, it comes through very quickly.
"This is why the depth of detail is so important. Also, what is in the book should be relevant and correct - authentic without being boring. Then the story has some weight."
On Leopard Rock is discreet about several aspects of Smith's personal life. His children aren't mentioned and there is only a brief reference to the polio he contracted at 16. "Contracting polio was a terrible experience," he says when asked why he treated it so fleetingly in the book .
"When I came to write about it in On Leopard Rock it felt like a bad holiday: the less said about it the better. The illness affected my ability to walk. When I was younger I was able to compensate for a withered leg, but these days it's caught up with me with a vengeance."
Certain figures dominate Smith's life and his memoir, notably his father and grandfather, both heroes to him - and heroes in general are clearly important to him.
"Literature throws us many great heroes," he observes. "Real life invariably outdoes them. From the times of Beowulf and Homer's Iliad and Odyssey, heroes and heroines have lit up an otherwise drab world."
Smith doesn't shy away from the subject of South Africa's troubled political history, writing with insight into its struggles, and is fulsome in his praise for Nelson Mandela.
As South Africa enters a new period of turbulence, Smith remains optimistic.
"I'm hopeful that the spirit of Madiba [Mandela] will see our country through the troubles towards a bright future," he says.
"It's a beautiful place to live inhabited by people of extraordinary qualities.
"Cape Town remains the prime cut of Africa and always will be, in my view. It's the jewel in a particularly special and unique crown."
As On Leopard Rock demonstrates, Smith is enjoying an active and productive ninth decade.
What's his secret for making the most of a long life? "To keep moving at breakneck speed - and slam into your coffin sideways when it's your time to go."
Lowdown
On Leopard Rock: A Life of Adventures by Wilbur Smith
(Bonnier, $45)