Whether in a notebook, spreadsheet, or by online website, most runners keep track of their mileage and workouts. Over the past few years I've used the website RunningAhead for my running log.

For an example, click the link to my training log.

RunningAhead is free, offers a lot of functionality, and is easy to use. If you have yet to start a running and exercise log, RunningAhead is my recommendation. Let me know if you have any questions.