That's a 6% drop the first half, and 1.3% the second, sounds perfect to me.
I would practice long repeats of at least a mile, and long runs of 10 to 13 miles down long hills, as others have suggested. Focus on running fast and coordinated with no effort. Give yourself plenty of recovery between sessions like this, i.e. once a week but no more often as your legs need to have the recovery. Go very easy for a few days after each hard downhill session to give your legs time to recover.
You will be very sore the first week, but this will lessen each week and you will get adjusted to them rather quickly. Use the downhills as your main session for the week.
Aim in the race for getting to the half a good pace, with very little effort and your legs in great shape.
Then gradually adjust to the lesser elevation, and increase the intensity to the finish. You should definitely get a much faster time than on the flat, which depends mostly on your preparation and also your strategy for the race.