A daily log can be as simple or as complex as you want to make it. Basic entries include date, miles run, and any special notes such as weather, hills, speed workout, etc. If your are into high tech, you may want to record heart rate. Logs can take the form of computer software, commercially produced versions found in bookstores and catalogues, or homemade versions on calendars or lined pads. Whatever works for you is fine.
Injury prevention can be a benefit of referring to a training diary.
Check the record. You may find that hill workouts did not agree with you in the past, and resuming them is the source of your problem. A sore knee may be your body's way of reminding you to ride the bike again, do your leg extensions, or buy new shoes. A log can tell you this important informatiom. When it comes to performance improvement, check the record to see what type of schedule worked for you in the past.