postgresql - Extract seconds from interval table / Cast record to interval?

Question

Im getting interval times via:

SELECT time_col - lag(time_col) OVER (ORDER BY whatever) 
FROM myTable where conditions

This returns a table like this:

00:00:38
00:05:10
00:02:05
...

I want to have the time in seconds like this:

 38
 310
 125
 ...

Here is my approach:

 SELECT EXTRACT(epoch from dt) from (SELECT time_col - lag(time_col) OVER (ORDER BY whatever) FROM myTable where conditions) as dt

dt should be the table with the difference times (intervals). However I get the following error:

Error:  Function pg_catalog.date_part(unknown, record) does not exist

So I have to cast record (the table 'dt') to interval? How do I do that? Or is this completely wrong? Sorry Im new to database queries....