I've been doing HIIT on a treadmill for about 20-28 minutes for several weeks (close to a month or a month and a week). In fact I ran yesterday on a treadmill for a grueling 28 minutes high intensity.
I just read up on Guerilla Cardio and ran a couple minutes ago. I feel that it's much better to run outside. I used my cellphone as a stopwatch and just ran on my street.
I don't like traveling all the way somewhere to just run so why not run just outside of your house?
Now that I have found this Guerilla cardio, it's going to be a while when I go back to a treadmill.
Honestly, you gotta go ALL out on high intensity. Do it on your street. You can't do that on the treadmill.
Trust me, it's definitely worth it. Just these 12 minutes, I felt a greater more deeper breath (vox2 or whatever you call it [epoc]) compared to 28 minutes of running high intensity on a treadmill.
Good luck and keep us posted mate.