Both have their role. Walking is appropriate for local short trips, while bicycles allow you to cover more distance, and is in turn superseded by transit in potential distance covered, while still being a low emissions mode of transportation.

Fewer CO2 emissions is a good goal if you are going to buy a car. Keeping it as long as possible is a better goal.

If the infrastructure allows for it where you live, going car-free is an even better goal for reducing CO2-emissions, and is only one of a long list of benefits of not traveling by car.

Barring that, voting and influencing politicians that can build infrastructure enabling more car-free lives is a good step in the right direction.