I could never get away with low low intensity cardio, id have to make it difficult as id be too bored to be able to keep on it...
Fasted works much better from my experience, but any is better than none if course..
My approach to this is to have a mix if the 2.. If I do 30 mins I'll do a full out sprint for 45 secs, every 5 mins, or id increase the level every 5 mins etc etc