Adarq.org

good question. mid to upper 160s for the marathon and upper 160s to mid 170s for the half. watch tells me threshold HR is 155, and that subjectively feels like a point at which running gets more costly. but then again, i went back and re-read davis's articles about LT1 and LT2 and he points out that HR% estimates are incredibly imprecise (or rather, that they vary so much from runner to runner that they're hard to use well). he suggests setting LT2 at 90% of 5k pace and working around that, which would be 4:12 based on my recent 19:07. that's quite a bit slower than estimates on lactrace (4:05-4:07) or coros (4:06). so perhaps i'll just stick with the lactrace paces, heat-adjusted, and not worry so much about HR.

If LT2/threshold is meant to be roughly what you can sustain for an hour, then your threshold HR has to be higher than your average in the half, right? (pace adrenaline/race day modulations). But yeah setting based on races and relying on pace is what Copeland recommends too (iirc the 3/6/10 min reps are roughly at 60/90/150 minute pace, which you can roughly calculate from here: https://www.finalsurge.com/tinman-calculator)