This is craigslist, not facebook: don't over-engineer what you don't need to over-engineer. You just look at already-made datasets that give the real world distance between two places, and remove search results based on that.

Never, ever, use google maps for this, and definitely don't use it "per query, as needed". The overwhelming majority of infrastructure doesn't change on even a yearly basis: use someone else's precompiled dataset, and update whenever they push an update.