I've been experimenting with something called "Salt Stick", they are in capsules like Enduroytes but are said to have a better nutritional profile. I had intended on taking one before the ride, and will try this next time.

It may well have simply been hotter than I realized and was tricked by the low humidity. Usually, however, that kind of problem presents in the latter half of the ride not the first...and I felt much better by the end of the ride.

Good thoughts everyone, and thanks!