I struggle with threshold intervals. My strength is 1-2 minute power. My mind wanders when hard intervals stretch out beyond 8 minutes.
It’s easy to forget what an “under” is supposed to feel like. It feels terrible, but is just barely sustainable. So I use HR to roughly gauge my perceived exertion.
I get up to around 190bpm on ramp tests and 175bpm on hard VO2max intervals. So if I’m 5 minutes into a threshold interval and ready to quit, but my HR is steady at around 170-175bpm, I know I can keep going.
Fueling definitely helps, especially with perceived exertion. I’ll never hit a threshold interval without plenty of sports drink.