Are you referring reverse periodization, ie Carmichael Training Systems? It's just a reverse of the traditional training season starting with the build phase then easing into base. It also assumes one has sufficient fitness. The other, also time-crunched, is to bang away at sweet spot.
If you call any interval at upper endurance/lower tempo pace or thereabouts as HIIT, then go for it. If you really want to do HIIT, for example Tabata 1E1 protocol, get some base miles in and save for a power meter. Your HRM cannot track the intensity need for the effort; each interval will be over before your heart can respond. Tabata is done at 170% of your power at VO2 Max. Getting down to 150% is still within the ballpark but questionable to not applicable for lower intensities. If you are new to cycling shoot for two or three years of base miles. If you are in good shape, aerobically from a different sport for example, maybe start structural training after the first year and incorporate HIIT 8-12 weeks out from you "A" event as part of the build phase. Do not do HIIT just for doing it, it should complement the demand for that event.
You need to increase your saddle time if you want to ride longer. Add 10% per week and just ride. If you want to go faster also, add in some tempo or sweet spot work. Water and some added pink salt is all I drink and one energy bar (a Nature Valley fruit & nuts or a few fig bars) should be enough. I don't normally eat for rides around three hours.