tbh at 320lbs with little or no cardio work prior id be inclined to say give HIIT a miss for now.
The reason i say this is that performed correctly its harsh even on the most ardent trainers.
Id rather see you doing steady state for the first couple of months, especially if your doing it outside, the easiest form of HIIT would be sprints.
At 320lbs, sprints on the concrete with little cardio experience is going to be hard on your joins / shins etc.