Thank you all for such helpful thoughts!

I had eaten about 2 hours before. I didn't weigh myself that morning, but my urine had been fine.

I'm not sure what you mean by feeling "de-fueled." When I need calories on a ride, I get muscle fatigue. Is that what you mean? Maybe you were overheated - did you take your temperature? Maybe dehydrated - did you weigh yourself or note the volume and color of your urine? Maybe mildly hyponatremic - what's your salt intake like?
Yep - just muscle fatigue and overall weakness...to me, it feels different than dehydration (something I'm fairly experienced with!). Now, the salt intake issue is entirely possible. I just looked up the hyponatremic (that's a heckofaword) symptoms and I have been bloaty ever since my 43 mile ride and had gained over 5 lbs in water weight. This wasn't atypical enough of me to worry about it (my weight fluctuates HUGE amounts and has since I was a teenager - usually 10 lbs at a time), but...could it be an indicator? Quite possible. I don't eat much with salt (I'm not an added salt person) and since I don't eat snack or junk foods...well...

So, today I got up early, ate breakfast 1 hour before leaving. I won't do that again, because I felt "urpy" (like I might possibly throw up) the whole way. I think I need 2 hours before, or something lighter (I ate greek yogurt with oatmeal mixed in, but that might have been too heavy for my stomach).

I drank 24 oz of water with Camelbak's "elixer" in it also. That might not have been enough time (I think I need at least 2 hours pre-ride). I didn't have coffee (usually 2 hours before). I also took some Clif shots with me, even though it was just an hour long ride.

After the first big hill, I had 3 clif shots and drank about half of my first water bottle. The hill seemed to go okay, but the muscle fatigue was greater than normal after it (hence the stopping for the shots). I noticed the lack of coffee as my motivation was severely lacking *laughing* I kept having to talk myself into riding farther and usually it's the opposite (I have to moderate myself otherwise I'll overtrain). By the end of the ride, I actually felt okay and could have gone farther, but I had to pick up my car. It was 92 degrees when I rolled into the parking lot after the ride at 9 am.

So now I'm not sure what the heck is going on. It's like my muscles are not getting what they need for the first 10 miles of a ride. It should not take me 10 miles to warm up!! I remember this feeling from when I ran cross-country (years ago) and it would take me a mile run before I could race. A mile or two I expect....10...not so much. I haven't been riding long enough or far enough to consider that a "warmup" Someday!

This only seems to have become a problem since the heat index went up to unbearable. So I'm assuming it's related, but it could very well not be...I'm open to that possibility as well. I'll up my salt intake and see if that helps....