Long slow distance work the current research says causes stress on the body which elicits cortisol a fat retaining hormone. My question is are you strength training? If you are not you are burning 25% muscle and not just fat which will slow your metabolism.
HIIT is a great add on for running training but not a cure all for anything, it is another training method. Here is a hyperlink to an excellent article on HIIT and how to incorporate it into your training. www.intervaltraining.net/hiit.html