Carl,
You've certainly got a solid enough endurance base to build some speed on top.
Have you timed yourself over that shorter distance?
If you can manage 8 miles in 1 hour (i.e. 8 consecutive lots of 1 mile in 7:30) I'd be very surprised if you couldn't go well under your target time over a short distance like 1½ miles.
If you can't manage it, maybe trying 3 or 4 sessions of
1 mile in 6:45, 2 mins standing recovery, 1 mile in 6:45
(or something similar i.e. faster than target pace, total distance greater than race distance, short recovery) between now and the time of your race should put enough speed into your legs for you to meet your target.
When running faster, concentrate on trying to maintain a faster legspeed than normal, rather than trying to do big, powerful ground-eating strides.
My £0.02p worth anyway