I had a ride like that once, and I whined and complained to my riding buddy (and when she got tired of hearing about it, I whined to myself) the whole time about how sluggish I felt, and how little energy I had, blah, blah, and and it turned out I had accidentally let a huge amount of air out of my rear tire when I was using this new pump I had, which I didn't discover until mile 19 of a 25 mile ride Ahem. Now when I feel like I am really dragging, one of the things I do, in addition to considering the food/water intake, is to get off the bike and check tires pressure and brake (for rubbing).

Was it hot? That's the other thing that can pretty much do me in...

[I'm not suggesting this is what happened with you, but it was a lesson that I learned the hard way!]