As I was reading this thread, thinking how tired I am after this morning's ride, I realized I'm never tired after my morning commutes (which are about the same distance that I rode this morning). I ate the same. They only difference was I worked harder this morning. So, maybe you just need to ride like a slacker, like I do on my commutes to work. See here:

http://www.bicyclefixation.com/slackers.htm

The hill advice is great.