I would say roughly 5-10 mins faster in a pure HM than in a 70.3 HM, but as everything life it is with a big "it depends", you can not really compare 2 races even if both are pure HMs. Also the heat depends, you will start fresh and cool into the HM and overheat only in the second half most likely, but normally 70.3s have nice aid and cooling stations every 15 minutes where you can cool down a little, haven't seen this amount of stations on a pure HM.

I would encourage you to go for the sub 1:30, that sounds reasonable based on your 70.3 times and it is a nice barrier to overcome!

Agree with Ben, gap closes from HIM to open with speed and experience in triathlon, but 5 - 10 seems fair from what you described assuming HIM course was accurate:)

I wouldn’t use my 70.3 Run time as a guide to benchmark my pure Half Marathon time. Rather, I’d be using a pure run time (a recent 5k? a 10k?) and something like VDot to extrapolate. The two approaches probably converge but I would have more confidence in the run-only based prediction.

If you are in the same condition/shape, you will DEFINITELY run sub 1:30 for a standalone HM in normal cool temps compared to a 1:37HIM in 100F!

I've generally found that my run times in triathlons are about one 'open distance' slower in terms of pace.

What do I mean by this?

In a sprint tri my goal is to do the run at my open 10k pace, in an Olympic I'm aiming for open half marathon pace, and in a HIM I'm shooting for my open marathon pace. I've found that if I bike properly and my run fitness is good, I can come very close to achieving this 'pace drop off.'

If you can run a 1:37 in a hot HIM I'm pretty sure you should be able to go under 1:30 in an open half marathon with decent mileage and speed work.

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In top form I can run a 1:15 stand allone HM. My best HM in 70.3 was 1:25 but started on the verge of cramping (got better/controllable during the run but always on the verge) and part of it was offroad. I would think I should be able to run faster in a 70.3 HM which the 5-10 minutes rule also confirms.

In 70.3 a lot is based on how hard you (over)bike offcourse and it is the net time that counts in triathlon.

5 to 10 mins faster holds true for me as well. FWIW in my 70.3 last year I ran 1:33, and a month later ran 1:25 in half mar. I surprised myself, my goal was to just break 1:30 but I shattered that. 6:30 per mile sounds nuts to me even now when I think about it but I would just say make a general goal and have fun with it and see what you can do!

2 years ago I ran an open HM in Jan. @ 1:37, My 70.3 run in April (similar hills) was 1:36. Then last year the same HM in Jan. I ran a 1:31, then the same 70.3 in April I went 2:05.

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My thoughts exactly. I predict 1:25-1:28