unset($temp);
$result = mysql_query("SELECT meta_key, meta_value
    FROM wp_postmeta
    WHERE `post_id` = $id
    AND `meta_key`
    IN ('length', 'location_city', 'location_state', 'difficulty')");
$temp[] = mysql_fetch_assoc($result);
print_r($temp); echo "<br>";

/* THE RESULTING OUTPUT

(
    [0] => Array
    (
        [meta_key] => location_city
        [meta_value] => Wabuska
    }
)