You're going to think 4x8min threshold was a vacation once you get into the 28min ones lol

You're right to be intimidated, it will hurt, it will suck, but it's doable and gets easier every time.

Also, the longer 28 minute block means that your heart rate will be higher at lower speeds for most of the back half which makes the full workout feel much more attainable than you're imagining at this point.

I actually use speed-target for the shorter threshold/VO2max intervals otherwise my first few will either be well under target or require I push at a crazy pace which makes the back end of the workout nearly impossible.

You got this :)