I have kept an exercise log since the 1980's. Once a diary it's now a spread sheet, but the format hasn't really changed. Just the basic data.
Date, Brief description including significant way points, Distance, Total Assent, Total time including stops, weather conditions.
For significant events I'll plan them in great detail pre event then right up trip reports and include pictures. Those are usually accompanied by a GPS plot, post event from memory, in the OS app. I find doing it that way I remember the route far better next time I'm in the area. I plot every walk on the OS App, post event, to pull the stats off, but for the vast majority I then immediately delete the plot.
Specific memories are usually a result of lessons learned or significant targets archived. Such as falling into the east dart river, mid winter, alone when it was -9C. whist doing a reccy walk for the the Dartmoor 600s for our walking group. First time I ever had to think, whist out walking, am I in trouble, can I sort this myself, if so what do I do next. Happily my instincts and experience of winter surfing kicked in, sorted it and lessons were indeed learned.