Thanks guys. I was already thinking this would be the nudge to schedule my annual (every three years) physical. It'd be awesome if the bloodwork would be a clue - "There it is! Your htfubulin is low... " but apparently, that isn't likely. Still, this feels like a necessary step. Scheduled for two days from now.
At one of the feed/aid stops on the course Saturday, a guy (as he handed me three paper shot glasses of pickle juice) said "I swear by potassium glutamate". So, if I combine all of the anecdotal advice so far, I should start the ride with mustard, pickles, hot sauce, Skratch, electrolyte tabs... and potassium glutamate.