I used to do this in high school when I was training for swimming. Hours in my own mind with nothing to really think about. Most of my conversations were in Spanish also, which was the language course I took in school. I ended up speaking Spanish fairly well and knowing a lot of proper grammar and verb tense in conversation.

When biking I can not honestly recall what I specifically think about other than, "Go harder you sally." My mind wanders and I think about everything but really nothing; the usual guys mind. When commuting I am fairly focused on traffic patterns, and where to go next and who is the next idiot that poses the biggest threat.