Offer to share some of this evidence if you feel they might be interested in finding out more about the impact of physical activity on disease prevention.
Help them to generate and articulate their own reasons, which may or may not be health-related and recognise they might not be interested in long term disease prevention.
Saying ‘if you decided to’ reminds them that they are the decision maker, not you. This helps keep the discussion open and active, focusing your role on providing support.