For me it is 150-160 bpm and 5-5.45 -ish min/km
Why:
4:50/km was my half marathon pace (165-170 bpm) and I am currently training to hit 3:30-3:38 marathon
So faster than 5:50-6:00 min/km I go out of zone 2 and slower than 5 min/km would be waste (not fast enough either for my specific marathon pace goal or threshold workout)
(Usually for zone 2 runs im at 6:00-6:15 min/ km lately)
You?