It depends. I know there is a century in Crossville that has the reputation of being flat and fast. If you're doing that one, just get up to about 60 miles on training and the adrenalin will carry you.
OTOH, be careful about the adrenalin. There is a tendancy to go out too fast and bottom out at mile 70-80. Go slower than you think you should, or stay with a nice cozy paceline, and have fun. We really won't care what your average moving speed will be.
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Momento mori, amor fati.