Ok. I don't know what you mean by your fitness level is not that great. Personally I wouldn't recommend anyone do HIIT who couldn't run at a decent pace for at least 20-25 minutes.
if you're real out of shape, you should follow a run-walk program to get yourself running. look around the internet for a run walk program, or just go out and run till you are tired, then walk to recuperate, and run again when you feel good... repeat. my approach isn't too scientific here, but it's logical I'd say.
if you think you're ready for intervals....there are numerous routines you can folllow, the basic concept is that you need to work very hard for a
short period of time and follow it with a rest period at an easier exercise pace
some possible routines are:
warmup for 5 minutes at an easy pace, run hard for a minute, jog for 2 minutes...repeat maybe 5 times...and add repetitions as you get in better shape
warmup, run at almost a full sprint for 30 seconds, jog for a minute...repeat...add reps
you get the picture....your workouts shouldnt' last more than 20 minutes I'd say, including the warmup. there is no need to add time to the workout, since you're always upping your effort (as you get in better shape, you run harder to exert that full effort on the sprint portions...)
ok, I hope I didn't confuse you.
tolga