My gut feel is that best way to achieve this is to take people out of their normal environment into one that better supports them in an environment that facilitates behavioural change; a retreat location where the attendees are fed a nutritious diet and encouraged to exercise with instructors present.
The jey problems with such a retreat are: cost; who pays (company or individual, government?); childcare responsibilities; work responsibilities; other responsibilites.
To address the cost and work responsibilities issues, the retreat could provide open plan work areas with internet connect, meeting rooms, telepresence, etc. This would reduce the cost to employer, potentially allowing the employer to fund the retreat. (Obviously this is only really suitable for the kind of worker who can work remotely.)
In terms of timescale, I have read that it takes 21 days to break or make a habit. Perhaps a month all-in. A subsequent month of support to embed new habits in real life.