Reading the OP, I didn't see one critical bit of information. Having "built up" to a 5 hour ride, what was your longest ride before that long ride?

It's possible you spent (lots) more time sitting on the saddle than you were used to. If that sitting time included long stretches going downhill, the resulting discomfort isn't surprising. Perhaps you might try "posting" (i.e., using your legs to lift yourself off the saddle) more in the middle of the long downhills, or putting a bit more power into pedaling (with the same result -- keister separates from saddle).