Yesterday was hot and humid in Austin, so you probably were much more dehydrated than you realized, especially after 3 hours. Also, until you get warmed up, your breathing and heart rate won't necessarily work together well as reliable indicators of your effort. To get out of my neighborhood, I have to go over about 10 small hills and my breathing will skyrocket before my heart rate catches up and I'm warmed up. But again, make sure you're well hydrated and if you start getting an elevated heart rate on a hot ride, you should rehydrate immediately, even if you don't feel thirsty.