A training diary helps you determine how well your training is working. It does so by enabling you to connect cause and effect, where the cause is your workouts and the effect is your changing fitness level. By looking back over the information in your training diary, you can determine whether you need to run more mileage or less, whether you need more speed work or less, and so forth. There is always a way to train more effectively than you are doing today. Keeping a training diary makes it easier to find ways to train better.