HR during Ironman

What should the HR for an athlete be during a half ironman? If for example when an athlete is doing a running workout he’s at 150-160 bpm, what should his HR be during the running part of the ironman? I’m confused since during the race you’re supposed to push harder and that means higher HR, but can you go higher than 160? That would be threshold for a 30 year old, right? Thanks!