Let's say you start a little conservatively so you don't go out too quick too early so you're about +30 seconds or so at the halfway point. When is it advisable to start making up that time in the 2nd half? Immediately with a few seconds per mile? Or wait until 20? Even later?
It seems to me that speeding up at halfway is risky because there is still a lot of race left. But if you wait too long then you might not have enough time to make it up.
Varies based on race geography, but once you hit halfway I would make a conscious effort to focus on form or running with / slowly catching people. The former works if you're alone, because 9/10 you will naturally pick up if you start ensuring your form is still correct.
All that aside, halfway to 15 is where I start picking it up, and by 16 I should be running my ideal goal pace or just under. Whatever my 16mi split is I want to be within 10-15s of it at that stage of the race. 20-30s if the second half of the race is easier / downhill.