Natalie, As this is my second half marathon I'm clearly well qualified to give advice
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What I would say is forget the time, set you objective as finnishing and enjoy the day and don't worry, with the times you've said you"ll probably be nearer the middle than end.
If you're limited with training time I would just slowly up your distance on LSR's (still got enough weeks) IMHO if you can do 17 - 18k in training you'll have no problem on race day. And don't forget to take it really easy the week before the race.
I'm sure there's other people on here who can speak from better experiance but that's what worked for me in my first half last year.