Every runner even slightly interested in performance should keep a running log. Unless you have an amazing memory, you’ll benefit from recording what you ran when before it slips out of mind. Over time your log will reveal patterns that can guide future decisions, such as how many days it takes you to feel recovered from a hard workout or which types of workouts seem to consistently lead to a given body ache. Logs are also great for reminding yourself how you trained before a given performance so that you can have objective information on what you needed to do to reach that level and insights on how to exceed it.
Include in your log whatever details from runs are necessary for you to be able to benefit from entries upon review. In addition to distance and/or time, that could include the type of course, the weather, time of day, what else happened in your life that day, etc. Beyond the basics, I always note anything that made the run different: Did it have a lot of downhills? Then that’s worth putting in so that if two days later my quads are really sore I have an obvious explanation. Did I do a track workout? Then my times from it will help me know what to shoot for in future workouts. Was it really hot for my long run, and did I then plug away as per usual on subsequent days? Then a couple weeks later I might have an explanation if I’ve been feeling flat. Was work crazy the last week? Did I sleep poorly? Did I run with Jim and go faster than usual? Anything from the run that might affect you on subsequent runs is worth recording.