How are you set up for the ride?
I just did my second commute today, roughly 5 miles in and 5 out for both work and class. Today was much better. I was dressed better from the start, had a new rack and rear basket to carry my back pack off body, knew my route better so I could anticipate hills and intersections. Other than my gearing getting very screwed up, skipping quite a bit and nearly throwing my feet off the peddles, it was actually a better ride. Next thing to change are my handlebars. Flat bars really start to hurt about 6-7 miles in.