Sort by location distance

I'm a little stumped on this one...

To keep it simple, I have city, state, and zip code. Some zip codes are missing entirely, which is making this difficult.

I need to sort the cities in relation to how close they are to each other. For example, any cities that surround Los Angeles, and any cities that surround San Francisco. I have cities all over the US, though. I'm not sure how to do this without going through one by one. In addition, I don't know all these cities. I barely know all the cities around where I live . It seems as though this could take a very long time.

Here's what I was thinking I could do. Regardless of the appropriate zip code, as long as I can get a zip code associated with that city, then I could assign it to the cities that have no zip codes. Then, sort by zip. It wouldn't be perfect, but I would think it save a lot of time.

What do you think?

Can you upload example?