Hey, @WattsUp. Yeah, those longer intervals are brutal, even if you’re good at the longer, steadier-state intervals up close to FTP. Couple suggestions in no particular order:
1 - Be flexible with your recovery weeks. You may find that 5 weeks of the HV plan is simply too much stress to adapt to in a week of easy riding, so consider breaking the 12 weeks into 3 weeks of loading followed by a week of recovery; maybe even 4 weeks on then 1 week off.
2 - Pick reduced versions of those workouts or add your own breaks. If the endurance necessary to make 20-30 minutes at a stretch isn’t there, break them into shorter efforts but only separate them by 30-60 seconds. These micro-recoveries can go so far in terms of productivity without utter annihilation by slow-fatigue.
3 - Trim off 5% from your Intensity if those longer ones are burying you and outpacing your ability to recover enough for the next long-interval workout. It’s better to have a little left over than find yourself utterly fatigued and dreading the next round of potentially fruitful effort.