Depends on the course.

I trained for my last century by doing a 30-mile ride the Sunday before, into the screaming wind as it was the day before a massive line of storms hit, then a 5 mile walk/jog, and then a 10 mile recovery ride. Took a rest day, then did the century. I was fine, but the course was mostly flat with a series of short yet steep bridges. To be honest, the 30 mile ride with the wind was worse than the century. Before the Sunday 30-miler, I hadn't done a serious ride for almost five months. (I don't recommend this training schedule, just using it as an example.)

BUT, if your ride is hilly and windy, you might want to consider the metric. You'll be surprised at the way wind can totally drain you on a long ride - hills end, but wind rages eternal.

Either way, eat every 25 miles or even less! And remember to hydrate! My ride almost ended early because I was trying to hold out at the very end and not eat - disastrous. Nothing sends you into a downward spiral like being tired, saddlesore, dirty and HUNGRY. Eating finally reversed 80% of my issues, and I finished up just fine.