Different Versions of HIIT

Well I decided to have a go at HIIT, or what I used to call sprint sessions a 'few' years ago.
The first thing I decided to do was find out the current trend of HIIT to ensure I was getting it right and that was when I realised a few fundamental things.
I have never studied the theories of this type of training. I did it a number of times when running competetively, it worked so I never questioned it.
There are a number of approaches to interval training some I feel rely on duration for intensity others go for sudden bursts of activity to attain higher shorter intensity.

I saw two that looked interesting and decided that a workout that included 5 minutes warm up and cool down and still only consisted of 14 or 17 minutes training wasn't enough so tried doing both back to back.

Now I am firmly aware that I have likely got one or both of these totally wrong, especially as I haven't done them before exactly as stated and I did them both. What I would like to do is post what I did, how I felt then ask what I did right, wrong and how it should have been done.

The basis of this is 20 second at high intensity, 10 at steady pace to recover, repeat 8 times with warm up and cool down.
I don't have a VO2 max monitoring system, I am running on the road so I translated the sytem to give it everything for 20 seconds and recover in the following 10.

I started well, warmed up, gave it beans for the initial sprint, recovered partly in the 10 seconds following, second sprint was pretty good but I found myself unable to fully recover in the 10 second sections meaning that my sprint pace kept dropping. Ironically I found myself feeling like I had fully recovered at the end of the last 10 second slot and was ready to run when the software I use told me it was cool down time.
There is an outstandingly strong chance I was doing this all wrong. But I don't think anyone is going to be able to give all for 20 seconds and recover in 10 for 8 sets without dramatically slowing throughout and I am not sure if this is what was intended.
Personally didn't really like this one but I would like to know from someone more knowledgable if there is a way I could improve this. Remember I have a GPS runnning app on my phone and am outdoors so VO2 measures would be largely impractical at best.

Because I am an idiot and didn't notice I had set the recovery to miles instead of minutes I totally messed up the start of the next system so the warm up consisted of 5 minutes run, 1 sprint, over 3 minutes jogging with minute wondering what the heck was going on, then ditching the warm up part from the app and trying again, hearing it say 'next interval 2 miles slow' realising I had been an idiot, changing it and starting at the first sprint after a few seconds jog.

This was the session which looked most like I had seen someone describing as HIIT. 20 seconds giving all followed by 2 minutes at a slow pace to recover. The recomendation was 3 of these with warm up and cool down.
This also gave VO2 percentages to be within but I decided give it beans and plod to recover.

I liked this one. Recovery was enough that at the end of each sprint I had been able to give so much that I was trying to pull in all available air and knew I'd been able to give far more in each sprint than in the equivilant Tabata session.
I feel I could have done far more than 3 of these but this is opinion only. At the end of the last sprint I felt dead but am assuming that to be the result of everything else before it not just those 3 sprints. Of course if I'd been doing this alone I may have been able to run faster in the three sprints but looking at my pace graphs from this it appears they were as consistant as gps gets.

The same as all interval sessions these was to work with not totally replace steady paced work, suggested frequency was 3 times a week.

What I would like is to see input from others who have done these and other forms of HIIT, specifically full on sprints to see what I am doing wrong, as well as how they found them.
I am intending to do these once a week, to accompany 3 longer runs. At the moment I plan to do Timmons but with more intervals, being potentially stupid I think I should go for 15 to give me a nice 20 minute interval session with 5 minutes either side making 1/2 hour. Comments on how smart/stupid/downright impossible this will be are welcomed. I would be doing it next week so plenty of time to let me know.
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