var lat1,lng1, first_check = false;

var watchID = navigator.geolocation.watchPosition(function(position) {
    if(!first_check){
        //first check is false, save these values
        lat1 = position.coords.latitude;
        lng1 = position.coords.longitude;
        //set first check to true
        first_check = true;
    }else{
        // next run, first check is true, so it gets to here
        // pass our coords to the distanceFrom function
        // along with our new set, each time it runs
        distanceFrom({"lat1" : lat1,
                      "lng1" : lng1,
                      "lat2" : position.coords.latitude,
                      "lng2" : position.coords.longitude
                    })
    }
});