When I'm doing strength training or in a class, I just think about what I'm doing at that moment, I usually have to focus. But running is so repetitive, I definitely need something to listen to. When I'm on the treadmill at the gym, I watch TV, usually the morning news is on at that time. If I'm doing a long run outdoors by myself, I have a small am/fm radio that I listen to. And if I'm running with friends, then I've got someone to chat with. If I'm not focusing on something else while running, I start thinking about being tired ... having something else keeps me distracted from the effort and seems to help me get into that "zone".
Interestingly, the only time that I DON'T have something to listen to is during races! But then, usually there's enough going on between the race course and other runners to keep me mentally occupied.
