Are runs of this length useful in 5k or XC training? I mean run by feel at what feels like 20-30 seconds per mile slower than estimated threshold pace. I find this type of run to be the most enjoyable for me as th pace is hard enough to feel fast, but still reasonably comfortable early on.
yes
Yes. Great, underrated workout
Isn't this called a steady state? This is also one of my favorite runs for the exact reasons the OP states. I sometimes work these into the middle of my long runs since it makes the run go by quicker.
Steady state efforts are great, I think of them as ball park like a marathon pace. It's nice because you can really keep smooth and fluid feeling while getting a really good training stimulus.
And what if someone does it in an over-under type:
20-40 minutes, 3 minutes under LT (-20sec / k slower than LT pace), 2 minutes over LT (+20 sec faster than LT pace)
?
Is it a better for LT (clearance / LT raise / other)?