I have a camera with the track to constraint added, and the camera is circling around a circle pointing at an object.
Please can someone tell me how to make the camera move in the opposite direction?
The animated path has a parameter called "Evaluation Time". This shows in percents where is the constrained object along the path. When you click on "Animate Path" button (at the constraints menu, you already did it) the curve gets an animation modifier what you can find in the graph editor. After you delete it, you can animate the Evaluation time as any other parameter. So you can increase or decrease its value by inserting keyframes at the appropriate frame.