There are people I know, or whose blogs I read, who keep really close tallies of their bicycle mileage. They will tabulate not only how many miles they have ridden in a year, but also how many of those miles were commuting miles, or errand-running miles, or off-road/on-road recreational miles, etc.
I, on the other hand, can usually estimate pretty closely what my total mileage is, but I can only give you a close-to-precise accounting of commuter miles (simply because I know my round-trip mileage and the number of days I've ridden).
It wasn't always that way, however. At one time, I had a chart on the wall on which I entered off-road, on-road, and even on-rollers miles, as well as elapsed time for each. I think that almost everyone who gets seriously into biking goes through this phase. Some grow out of it, others keep detailed records for 50 years.
Back in the late 80s, Val and I went out to Oregon to visit with Joy and Steve. I took my bike (another couple of stories).
One day, when riding up a particular mountain, I forgot to start my cyclometer so that it would register mileage and time. I met Joy, Steve and Val at the top, and asked Steve what the mileage was from the point where they had dropped me off.
I spent most of the following evening with my bike propped up on a couch, spinning the front wheel until the computer registered the 11.2 miles I had neglected to clock. It meant very much to me, at the time, to have every mile ridden registered on my odometer.
Now, I have a lot more miles under my belt, and I have realized something: It's not the mileage that's meaningful, it's the ride. The experience is much more important than any numbers attached to it. Although, I do still like to rack up 100 miles rides, and such But, the actual distance is less important than how much fun I had, that day.