SQL Server : Tracking a slow running query on completion

Am trying to work on script for slow-running queries. I understand that we can get the list of slow running queries using sys.dm_exec_requests. Say, I have a query Q1 which is running very slow, say, 1 hour. Till Q1 is running i.e. executing, I can track it using sys.dm_exec_requests DMV. If I understood it right, once Q1 is completed, its statistics will be no more available in this DMV. Instead, we need to look at sys.dm_exec_query_stats. But how do I get the statistics of that specific query Q1 which had been slow-running ? Meaning how do I relate the data I earlier got in sys.dm_exec_requests and sys.dm_exec_query_stats?