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
	}
)