In my experience, it's probably better to undershoot (a little slower than whatever "exact threshold" is). Generally that sort of work isn't meant to be super hard, and more volume at a threshold-y pace is better than a too-hard effort you may have to cut short.
I've definitely had prescribed tempo run paces feel too hard, and slowing down helped manage that. My theory is because I wasn't running enough volume and wasn't aerobically fit enough to scale, say, my 5k or mile equivalent to an hour effort. Basically I'd suffer more as the distance increased because of how I was trained then.
Slow down (a little) and run more for that type of workout prolly. Intervals might help.