I know what you mean. I didn't feel "warmed up" for my ride until I was 2/3 of the way done (on a 20 mile ride!). Everything felt slow and heavy and not effortless at all. At the end it turned out I had shaved 4 minutes off a time recorded over a month ago, but it was still not a fun ride. The last 5 miles felt better than the rest.

What I did was eat about an hour before the ride (I normally eat 2 hours before so wondering if this didn't help...) and drank a lot of water. I made sure to drink water often on the ride. Then once I got back I ate some crackers and drank Hammer Recoverite. Today I am feeling pretty good and thinking about going out even though it is even hotter today. Tomorrow is supposed to be even worse!