-
This is a fairly recent run where I was running at what I'd consider to be threshold: https://www.strava.com/activities/264087468/overview
Which roughly agrees with your forecast of say 165bpm for a half I think.
Now, the next question is whether running a half on a 10% average gradient will only add 45 minutes to the time I ran yesterday, that's just over an additional 2 minutes per KM.
Dammit-
It shouldn't add that much time, if you are strong, but you haven't mentioned that you're doing it after riding your bike hard for a period of time as well or are you just doing the run this year?
After riding at 78% of FTP for 3 hours in Rimini my half marathon time (on a flat course) was 17 minutes slower than my best time this year - although some of that was due to cramping rather than just fitness....
Sainsburys_Ed
How are you calculating your threshold HR? As far as the textbook goes, lactate threshold is about the sort of intensity you can maintain for an hour's all-out effort, so unless you're Mo Farah, half marathon intensity will be a fair bit lower. Looking at my own figures, roughly: max HR 200, threshold 180, HM 175. (Marathon 168.) I'd say your HM HR could be around 165, although for the purposes of a race if you wanted to be a little conservative you could start out around low 160s and see how it feels, pick it up in the second half if you're feeling comfortable.