Use the google matrix API to get the distance in miles from one location to another. This requires the google API Key somewhere in the spreadsheet.

get_mileage.js

/**
 * @OnlyCurrentDoc Limits the script to only accessing the current spreadsheet.
 */

/**
 * Take two string addresses, and use the google matrix distance API to
 * fetch the mileage (distance) between the two.
 *
 * @param {String} origin An Address that resolves in Google Maps.  The Origin.
 * @param {String} destination Another address that varies from the first and
 *                 resolves in Google Maps.  The Destination.
 * @param {String} apiKey The API Key to the Google Matrix API, see:
                   https://developers.google.com/maps/documentation/distancematrix/intro
 * @return {String} the mileage text distance between both addresses.
 */
function GET_DISTANCE(origin, destination, apiKey) {
  Logger.log("Fetching distance between " + origin + " and " + destination);
  var matrixHost = "https://maps.googleapis.com/maps/api/distancematrix/json?origins=";
  var encodedOrigin = encodeURIComponent(origin);
  var encodedDestination = encodeURIComponent(destination);

  var requestUrl = matrixHost + encodedOrigin + "&destinations=" + encodedDestination
          + "&key=" + apiKey + "&units=imperial";

  var resp = UrlFetchApp.fetch(requestUrl);
  Logger.log(resp);

  var jsonData = JSON.parse(resp);

  return jsonData.rows[0].elements[0].distance.text;
}