The aim is to sign up more than 100,000 people, with daily stats recorded digitally.
If the target is hit, Sir Brendan says it would count as the biggest marathon in history.
He hopes streak culture, the habit forming behaviour as seen on Snapchat and Duolingo, will help people stick with the challenge.
The health benefits, and potential NHS savings, will also be significant.
“If someone walks 30 minutes five times a week, they could gain up to four extra years of healthy life,” he says.
Sonia Pombo, head of research and impact at Action on Salt & Sugar, says: “Encouraging people to build regular movement into their daily lives can support better health, and making it simple, achievable and rewarding may help more people get started.
“But we cannot rely on individual behaviour change alone. If the government is serious about improving the nation’s health, particularly for children, it must pair initiatives like this with stronger prevention measures.”
Full details of the voucher scheme will be released in the coming months, along with information on how to sign up.