How to keep a simple running log?
Choose a logging tool, for example the daily spreadsheet, a hardcopy sheet that you place in a visible spot (this is the most effective method), or an online tool. There are several options for a runner training log depending on how well you know the technology. If you prefer old-fashioned paper journals, there are plenty of options available, including the full runner's daily journal, one of the classic training journals for over 30 years.
If you prefer a high-tech approach, there are plenty of standalone apps for your phone (like Running Log Free) that will do the job.
A run log is just anything that records your progress over time. As you already know, just running in your head and tracking your progress can be confusing.