Ever walk on a track? It’s kind of fun, in that you get halfway around and even if you want to stop, you can’t, because you still have to walk back the other way to get to your car. Plus having a mindless route — it’s walking around in a circle, for heaven sakes — makes it easier to think about other things besides where you’re going. You can notice the trees, or the weather, or the other people exercising around you. And of those, there are always certain predictable types: the parent maneuvering the huge stroller like a DC-10 coming in for a landing, the athletic competitor zooming past on a training run, the elderly person listening to the ball game on their progress-be-damned transistor radio as they walk slowly around in a windbreaker and sensible slacks.