To do that you would need to have a database full of locations or do the math to do all the calculations based on the longitude and latitude. Without some type of database you’d need to get all the posts/fields and do the calculations 1 at a time? Sorry I can’t give you more help than that https://stackoverflow.com/questions/27928/calculate-distance-between-two-latitude-longitude-points-haversine-formula.
I use the pro/gold version of WP Google Maps, it does all of this for me https://www.wpgmaps.com/purchase-professional-version/
