I've kept a daily journal since the year after I graduated from the university (35+ years). And if there was one message I would give to Earlier Me, it would be to have started a journal earlier. But then I look at the bicycle mileage journal I started while race training in 1983, and realize I probably wouldn't have had the discipline. It started as a paper "time manager" calendar/notebook at the strong suggestion of my employer, and transitioned to a Word document around 2010. I've scanned in all the old paper journals for reference and archiving.
As part of this journal, I log all my rides, whether to the corner or a full centrury. Having a record makes a big difference between able to recall "um, some time in the spring in the mid-90s" and 'Saturday, May 25, 1996, at sunset, on the tandem, along the beachfront path."
I've led a weekly ride for the local bike club for 33+ years so far. We meet at the same location at the same park at the same times, so it's easy for all the rides to blend together. But I try to keep the routes to our restaurants interesting and varied, and log any noteworthy or unusual occurrence. Same with my weekly commute to the co-op - there are completely humdrum rides, and others that are memorable for certain... reasons.
I've also kept a spreadsheet of bicycle mileage for this time span. Keeps track of incoming and departing bikes, notable rides, and reminders for maintenance.