not so fit, you have to look at it differently. If it doesn't go well on the day, blame the race organisation, blame the other runners, blame the course, blame the lack of toilets at the start. Or, if you really think it was your training, blame the early nights, the overwork, the family that just doesn't understand, the dog that you tripped over spraining your ankle. Just don't blame yourself, you're doing everything that you can!
Honestly, you're going to have a great day out, doing something you love, alongside thousands of other people who love doing the same thing you do. You're going to get fed, watered, and cheered along the way; laugh at strained photographs online afterwards; and sign up for every other race you can find!
Who needs a PB? They're easy. I got one in the very first Half I ran! 