I have been doing my own kind of HIIT lately, I am wondering if this is helping. Basically what I am doing is running hard till I reach about 180 +- 5 Heart Rate. I then walk and try to bring it down as quickly as possiblt to about 140-150, I then go back up to my 180. I do this 6 or so times, and I usually have done a little more than a mile by the end. Is this an effective workout?