Just go out and ride a 75 mile ride and see how you feel the next day. If this is the course you are selecting to ride for the century and you aren't too sore, if all else is a "go" you will be just fine.
Base miles...I had about 100miles per week and ramping up. Long distance ride was 81miles and the heat just wiped me out. I was well fed and watered but the engine just gave out. Next day my legs were fine and I knew I was ready next time around.
When I rode the century there was a steady 12-15mph headwind that caused the last 15miles to be a chore. Next day I felt great but sure didnt have any inspiration to get on the bike for another 4-5 days.