I don't think it's possible to extend the length of the default ringtones, (unless you use a sound editor).
The best thing to do is search for 'free ringtones' on the Internet or in the Google Play Store, although you will find many aren't free so you may have to search to find free ones. Often they are quite short so again you may have to search a while to find a lengthy one.
You can also change notifications as well ... I use the 'Carpenters' version of 'Mr Postman' for emails.
You can also set different ringtones for different people in Contacts ... useful if want to know who's ringing if you want to ignore someone!
Hope this is useful and you get it sorted.