Generate featured images old posts

You can look into this post: http://ken.ph/first-image-of-wordpress-post-as-thumbnail/

Or if you’re comfortable with MySql try using this (obviously check it on local database first):

INSERT into wp_postmeta (meta_value, meta_key, post_id) 
SELECT DISTINCT(ID), post_type , post_parent 
FROM wp_posts 
WHERE post_type="attachment" 
AND post_parent !=0 
AND post_status="inherit";
UPDATE wp_postmeta set meta_key = '_thumbnail_id' 
WHERE meta_key='attachment'