I find that I improve by using hiit a couple of times during the week and riding 50 odd miles one of the days at weekend. I use the weekend ride to asses how well the weeks hiit has been. My 30 second "burst" has to be so hard as to only just be achievable,at 15 seconds I can't spin the pedals above 60 but just push on barely able to turn the pedals at the end and feeling like I'm suffocating from lack of oxygen,fairly unpleasant for 30 seconds or so of my recovery time ,but it passes.
It works for me,have a look at the work Mac master university in Ontario has done on the subject ,start off with maybe 3 reps with 4 mins in between for recovery and reduce the recovery time each week ,just give yourself time for your breathing to return to near normal .
Just my opinion,