I'm all for designing for walkability, but this is a case study in failing to understand audiences. If you make this not just a possibility but a requirement of walkable cities, many people won't want to live in your "walkable" cities at all.
There are much better arguments possible here. For instance, you could promote CSAs for regularly delivered farm-fresh produce, or establish efficient grocery delivery services that use one vehicle to deliver to many many customers, or other potential options that don't involve regular car trips for grocery shopping and don't involve spending an appreciable fraction of your day shopping.