"We really need to encourage everyone, but especially children to get up off their bottoms and do a little bit of exercise," she said. "It has to be something they will enjoy to lure them away from the TV and computer screen. Football, tennis, netball, running, dancing or playing rounders are all great fun."
Ms Kelly, who began her career as a researcher for BBC Scotland before switching to TV-AM 18 years ago, ran this year's London marathon in five hours and 57 minutes and raised pound;10,000 for charity.
Half the money she raised went to the British Heart Foundation and the rest was divided between three Scottish charities and Maplewood school, in High Wycombe, Buckinghamshire, which bought a hydrotherapy pool for its disabled pupils.
The 44-year-old presenter said: "No one is asking kids to run a marathon and glower at a lettuce leaf. They can still have burgers and chicken nuggets by just cutting down on the crisps, fizzy drinks that are full of sugar, cakes and sweeties."
As a child in Glasgow, Ms Kelly was always outside: "I spent all my time playing games like kick the can, football and two-man hunt, which is like hide and seek, only scarier.
"We were outside all the time and full of energy."
Junk food was rarely on the menu at home: "My mum made brilliant home-cooked meals and we did not eat all that much rubbish. Junk food certainly was not banned, but it was not so readily available.
"Things like crisps and Coke were a big treat, to be guzzled watching Dr Who on a Saturday night. Now sweets are no longer a treat for some kids and they eat far too many.
"The Get Active campaign is a cracking idea. Good luck with it," she said.