I'd also like to mention to make sure your brakes are not rubbing on your tires and that your tires are fully inflated. You should be pumping your tires every time you ride.

I did a century once where I was just miserable for the first half. I couldn't keep up with people I should have been able to keep up with. I was struggling and depressed and literally cried. Finally it occurred to me that my front tire was low. Turned out I had a slow leak. After I fixed it, I was going faster, but still seemed to be having some issues. I then realized my brake pad was rubbing. OMG!!! After that I was AMAZING!!!!!!!!!!! My over all time was not nearly what I had hoped for and knew I was capable of, but I was SO happy!!! I just kept saying, "I don't suck!!!"