Вы можете использовать distanceFromLocation: метод CLLocation , который возвращает расстояние в метрах между двумя CLLocation's .
CLLocation *currentLoc = /* Assume you have found it */;
CLLocation *restaurantLoc = [[CLLocation alloc] initWithLatitude:restaurantLat longitude:restaurantLng];
CLLocationDistance meters = [restaurantLoc distanceFromLocation:currentLoc];
// Go ahead with this