Found this in this month’s Runners World. A useful guide to picking realistic target times based on your shorter run pace.

My current 5K averages at around 30-32min so that gives me a HM targe tof 2:18 hr to 2:27 hr for my HM. Which sounds about right — when I last did a 17K run, I did so in about 2 hours, so add an extra 3K onto that (and the fatigue), which put me at around 2:30 hr.