The stuff your brain throws at you is amazing. Sometimes it's like cleaning a garbage disposal. At its best, it's elation. What's really interesting is that you can have several planes of moods, thoughts, and even physical states. Like developing orchestra listening skills, you can get really good at tuning in, or tuning out, certain sections of yourself.
First run of the season, Water to Thrive, which helps dig wells in Africa. It was a sunny Texas day and my legs felt great through the end. I walked a bit thanks to that heat, but still had a great time.