Jeff Galloway, from the Nathan log book:

Why keep a log?  For one thing, it allows you to relive the good times. Somewhere down the line,you might gain great enjoyment looking over your past running experience. What may seem like the recording of trivial data can tomorrow be the basis for analyzing progress or meeting challenges. Reflections upon these periods when things went well can often lead to future success in both training and racing.

But a second and perhaps more important reason for a log is that it allows you to analyse what led to an injury or less-than-satisfactory performance. The log will help you learn from your defeats, since it is your running history. It's the only way you will be able to analyze and then interpret your personal running data. And even if you don't find your mistakes, you will learn to include more significant data in the future logs. Past problems can help you make powerful changes - if you have the right information written down. The account of your own successes can often lead to more successes.

This log includes tips on speedwork, hill training, injuries, rest, mileage, and racing, from some of the world's best runners, a flexibility program from the country's best stretching expert, plus other helpful features like a distance/pace chart.

This may be the best $9.00 you spend on your running this year.