I keep a Google Doc spreadsheet log of all my training sessions: Mileage, any speedwork mileage, time spent in strength training plus other active/intentional physical effort, and estimated distance walking.
I also track known lifetime training mileage, and a self-created stat called Chops.
Chops is named after the musician term chops, which describes a performer’s current musical skill. Similarly, my Chops number provides an estimate of how many miles I can comfortably run at full strength over the following week.