If there was a defining moment that forever changed my perception of physical activity, it was a trek to Everest Base Camp. Undertaken in 1975, at a time when adventure travel was in its infancy, my partner and I were to traverse more than 400 kilometres of the most challenging terrain on the planet - with one Sherpa to guide and carry for us. But our careful preparations, strong on providing appropriate food and shelter, were woefully short on preparing our bodies. In a ludicrous, token attempt to accustom ourselves to the loads, we had circumnavigated the green at London, Shepherd's Bush - once. For a woman who was later to address her physical fitness with something approaching religious fervor - it was completely laughable.

The miracle was that we made it to Base Camp and came back in good enough shape to tell the tale. My memories of those weeks should have been of the warm, friendly people and the majesty and magnificence of their environment. Instead, I was focused on the crippling pain in my knees that made me long to walk uphill, no matter how steep or long the incline, rather than descend. Other pain came and went, as muscles, unaccustomed to physical activity, let alone of such demanding nature, screamed for respite.

But back home, something had shifted. The memories that remained were, surprisingly not of the pain, but of the exhilaration of pushing myself to my physical limit. I started running, and for the next decade I ran ten or eleven kilometres every morning of my life. I was addicted - to the endorphin hit, to a new body shape and to the liberation of disregarding caloric intake! Most mornings I saw the dawn break, I was on first name terms with the garbage men and nodding acquaintances with other early morning exercisers.