High Intensity Interval training (interval) studies are always very short in time duration, maybe 6 weeks on average. Riders who follow them will get fitter quicker and plateau very quickly (maybe 6 months)

HIIT is great if you want hormonal imbalance, CNS imbalance, and to develop a-fib.

The best way to exercise is just to consistently do a lot of long rides of 1-3 hours at a moderate and enjoyable pace. About once a week or so, do some very hard efforts like hills or intervals. In a few years, the difference in fitness to a HIIT program will be huge. Also avoids the tedium of trying to adhere to artificially constructed training zones.