Warning: I ramble....alot
You'd better believe it. For me, 3 miles is a light day. I'll usually go for 4-5 miles, which will usually takes about 40 minutes. I'm a total cardio whore, like the Paris Hilton of Treadmills or something.
IMO 18 minutes of cardio is hardly any cardio (how many calories do you expect to burn?). If 30 minutes of straight up HIIT is too much for you (don't feel bad, it took me years to build up to this capacity), it's not uncommon to do 15 minutes of HIIT followed by 15 minutes of normal cardio.
In all actuality, you are more likely to burn (on average) more calories with normal cardio than with HIIT (in the same workout duration). Before someone calls me out on the above statement, I'll derive it from 2minute segments from each style:
HIIT: 1 minute @ 12mph 1 minute @ 3 mph = Average speed= 7.5 mph
Reg: 2 minutes @ 8mph Average Speed = 8.0 mph
Since Work= Calories expended = Force * Speed * Time
Force is related to your bodyweight (which is constant). Time will be constant (we are comparing two workouts of the same duration) Thus, the average speed will be the deciding factor in determining how many calories you burn.
That being said, it's not that HIIT burns more calories per hour, but more calories from fat stores due to the way it recruits energy.
I hope you're still awake, and this makes sense.

thesinner