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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-
Yes, still doing the ride - at I would think 92% of FTP for an hour and a half, as much less than that won't get me up the slope!
Pretty confident that my HM pace is around 1.35-1.40 if I were to give it the beans a bit.
So the question is how much the ride will have taken, and then how much the gradient takes on top of that.
I'm not really expecting anything from the ride this year, it's all about the run, but I'd like to get as close to 4 hours as possible.
Dammit
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.