With such a topic so broad we truly try to cover the basics from all angles in this group. Nothing too big or too small. Nutrition is as significant if not more as exercise is to reaching your goals so learn all you can.
Yes start doing HIIT.
Change from slow to fast run (and the opposite!). Slowly you'll be able to increase the time of running fast versus running slow.
Good luck!
-------------- - Nina :o) Les Victoires éternelles sont celles du coeur.
Head for the hills - either one that you can run up and down (similar to HIIT as Philia2 suggested) or a park with a trail that includes some up and down - even a slight incline for a few miles will help.
Like asimmer said, try to get a track workout in once a week.