Lot's of good points all around.
On gearing: I don't think this is my issue; I've climbed this hill three other times in the last 5ish weeks, and felt increasingly confident on it using the gearing I have. Long, tough climb for me but no stops or notable issues.
On pacing: surely something I need to take more seriously. The first climb of the day was Old La Honda, where I was only a minute behind my PR. That's probably just a bone-headed way to start a 70 mile day, given I've never ridden more than 50ish on a single ride previously.
On fuel: I guess I need to get my ass out of bed and get started on digesting breakfast earlier. I think I probably did okayish in terms of the fuel I ate on the ride and how I paced it out, but maybe could get in a better rhythm of frequent fueling.
On hydration: my first pee was hours after my ride, and was moderately yellow. I'm surprised to say it, given how much I drank and how it compares to how much I usually drink on a hard ride, but I think dehydration definitely came in to play here. I wonder if my time in the sun on the coast took more out of me than I realized. I'm probably just a guy who needs more than the standard recommended amount of water for these types of distances. Like I said, I'm a sweater.
On what was actually going on during that hour of fail: I think what Carbonfiberboy is describing about a low blood sugar event pretty much nails the experience I was having. I am still curious about what you guys mean when you distinguish between this and a bonk. When I google bonking, the internets seem to be generally describing bonking as relating to glycogen storage and hypoglycemia. I guess you're distinguishing between general hypoglycemia and the more specific immediate event of depleted glycogen stores in the leg muscles? I'd be curious if anyone wants to clarify that further. Either way, I was pretty damn useless! But basically, once I got back down to Woodside to fill my bottles again, I was able to get something going again by doing exactly what Carbonfiberboy recommended: rest and hydrate until my legs didn't feel completely worthless any more.