This is not a problem that was created by individual actions, and it won't be solved by them either. The bodies responsible for planning our infrastructure (roads, sidewalks, office buildings, etc) need to make "active" choices the most convenient. This could be as simple as making the elevator a little bit hidden (but still available for those who cannot use the stairs) whilst making the stairs look more inviting. Or ensuring that a direct and safe network of sidewalks is always available in our city centres. Or prioritizing pedestrian phases at traffic lights.