Yes. HIIT will help with long distance running - however if you are training for a long distance event, you should include one long distance day in your cardio training.
Try this:
1. HIIT (AT) training
2. long -distance steady state training (ideally go longer than the distance you will be tested at - for example if you are tested on 5 miles... do 7 )
3. Rest day
4. Longer interval training
5. HIIT
6. Rest
7- onwards.. repeat cycle... then taper off a week before the event..