What have been your typical length of rides prior to the century ride?

If you've only been doing mostly 1 or 2 hour rides and your saddle has been fine for that, then maybe your butt just wasn't use to being in the saddle that long. If you keep doing long rides more often it'll get use to it.

One thing in play here is probably as the ride gets longer and longer, you put less power into the pedals. So as your power to the pedals goes down you sit heavier.

Whether or not you need to swaps saddles only you can say for certain. But it might just be that by the time you find that magical saddle, it's not the saddle, but that your butt has gotten use to the new parameters it must operate under.