Her warning comes amid growing concern about the damage that increasingly sedentary lifestyles are doing to health.
Experts say it takes an hour of exercise a day to undo the damage caused by a desk-bound office job.
Dame Sally said: '"Everyone can become more active by making little changes to the things we do every day.
"If you can go for a run about the park- great. But if you are pushed for time, something is always better than nothing."
Dame Sally, who caused controversy earlier this year when she told women to think about their risk of breast cancer every time they fancy a glass of wine, urged people to follow her lead.
"At home I do little things that I know will make a difference, like walking to a nearby shop and carrying bags home, or even getting off the bus a stop early," she said. "When I am at work I try to take the stairs to my office over the lift, and not make too many trips to the biscuit tin."
Professor Kevin Fenton, of Public Health England, told The Sunday Telegraph: "We've gone from hunter-gatherer to grocery-carrier to simply signing the delivery note."
Carrying shopping works off calories and also builds muscle, which is vital for balance and independence in later life.
Steven Ward, executive director of charity ukactive, said Britons should learn lessons from the past to prevent them "limping towards an early grave".
He added: "Modern living has made us more inactive than ever before. Where once we would walk around the office talking to colleagues and carry home our groceries, today we fire off emails and have Tesco deliver food to our door."
