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Good work! I think there's a bit of trial-and-error when it comes to getting the intensity right, because one of the aims is to be able to complete a full session with each hard bit being the same pace. (This really helps get used to race pacing.) But you should also be able to do it whilst keeping good form. If you're consciously changing form in a forced effort to up the pace, try to ease off just a touch, and concentrate on keeping your cadence ticking over nicely. Over time, getting used to doing interval and tempo running not only gives you a marked improvement in cardiovascular fitness, it also helps to improve your running economy, as your running form naturally adapts to operating at a faster pace as comfortably as possible. Another popular way of thinking of "tempo" or "threshold" intensity is comfortably hard, but hardly comfortable.
I could only do three sessions, not four. It was meant to be 7/7 on/off but I could only manage five minutes for the fast part. I felt I really needed to focus on making my strides longer and pumping my arms a bit to get the speed going. But maintaining that for any length of time beyond 5km or so seems really daunting.