In reply to spacey: Depends when your previous Half PB was set.
I'm by no means a fast runner 4:30 best marathon, 1:54 best half. The long runs I've done in marathon training have had a massive impact on my heart rate at the kind of paces that I was running halfs last year. I'm fairly sure I'm capable of sub 1:45 but will see how I do on Sunday in an undulating trail half.
Possibly my problem is that I don't run flat road races. I ran Peterborough half last year at 9min/mile in prep for the Rutland Water marathon. It was unbelievably easy. Rutland was not, by mile 10 running at 9min/miles, against freezing wind, rain and 'undulations' I was struggling.
I held on right to the end though, some parts running along flooded paths where the water was halfway up my calves. People were finishing borderline hypothermic. A brutal day!
Essentially what I discovered was I need 5 aims for a race. If my first aim was lost then I have a second etc. This way I'm not lost if the first aim becomes unattainable.
1. Sub 4 - theoretically easily achievable in ideal conditions on an undulating course.
2. Sub 4:15 - if sub 4 isn't happening then drop pace to recover and set new target.
3. Keep running and finish in whatever time this produces.
4. Run 100m, walk 20m.
5. Walk.