Here are a couple other thoughts regarding your post.
The short steep hills are what we normally include in the early base period, after the transition period. We call them Hill Sprints. We normally keep them under 15 seconds and they are done at 90+% of max intensity with long rests to allow for complete recovery. These have a primarily “strength” building training effect and would not be considered part of High Intensity Interval Training program. HIIT will normally look like extensive repetitions of 30sec to 8 minutes at intensities above the anaerobic threshold separated by intervals (hence the term “Interval Training”) of (usually active) recovery that vary from 100% of the work time for the short work periods like 30sec and can range down to as low as 25% of the work period for long repetitions. The recovery period and whether it is active or passive will affect the energy system being primarily targeted. Shorter rests means a more aerobic training effect. Long rest allows more full recovery and so enables higher intensity/anaerobic work to be completed.
An important thing to understand is that; it is the aerobic metabolism, even thought it is not providing the majority of the ATP for that HIIT work, it is performing a vital supportive role and it restores your body back to homeostasis during the HIT intervals. The higher the aerobic capacity of the muscles the faster will be the recovery, both within and after the workout. This allows one to do more high intensity work and gain from the attendant benefits.
Let me be clear:
High intensity training plays an important role in the training for all endurance sports. But it must be a supplement to, not a replacement for, aerobic training. The relative importance and the methods of HIIT used are dictated by a number of factors:
1) The event you are training for. Shorter events have greater reliance on HIIT.
2) Your personal aerobic capacity. If you are aerobically deficient then it would be short sighted to concentrate much efforts on HIIT. Yet this is the standard prescription that comes from the fitness industry.
3) The HIIT needs to be event specific to a large degree. If you are training for the local 5km road race then leg speed and lactate tolerance training needs to be employed for best effects. If you are training to climb the West Buttress of Denali then you do not need leg speed nor lactate tolerance training (these would actually be counter productive) and you need event specific muscular endurance training.
For more background on the underlying physiology of “why” these things are true I recommend reading the Physiology chapter of Training for the New Alpinism
I hope this sheds some more light on to my comments from yesterday.