HIIT (High Intensity Interval Training)

So I went to the gym yesterday having decided to give HIIT a try.  I found one program that suggested the following routine for beginners:       
Beginner's Layout
Minutes 1-4 (Warm-Up)Jog at about 50% effort
Minute 5 (Workout Interval 1)Sprint 30 seconds at maximum effort Jog/Walk 30 seconds
Minute 6 (Workout Interval 2)Sprint 30 seconds at maximum effortJog/Walk 30 seconds
Minute 7 (Workout Interval 3)Sprint 30 seconds at maximum effortJog/Walk 30 seconds
Minute 8 (Workout Interval 4)Sprint 30 seconds at maximum effortJog/Walk 30 seconds
Minutes 9-12 (Cool-Down)Jog at about 50% effort
I kinda forgot the pattern and did the warm-up walk for 3 mins, and started the :30 sprints/:30 walks at 4, 5, 6, 7, 8, 9, 10 mins, then I walked for the remaining 9.5 mins for a total of 20 mins.  Wow!  I was running my ass off, and the :30 walk goes by very quickly, but so does the :30 run.  I think I only burned around 140 calories, but it felt great.  I hope to build up my endurance to do :60 sprints soon, but all things in good time.  Baby steps!  Going to do this one a couple more times, then maybe try to do a minute on/a minute off.
I never pictured myself as a runner, but it makes me feel so STRONG!  I love it!