For me, I have a CatEye Mity3 mounted on my commuter and I just read off the odometer every Jan 1 (also for our cars). I have an email thread to myself with all of these odometer readings.

That computer used to swap between the commuter and my MTB with a second mount (MTB is a 29er so I stopped even switching to the 'B' wheel diameter setting), so recreational trail riding was also included (when I remembered to swap the computer over). But on my 'new' MTB, for some reason I was unable to get the computer working. I didn't care enough to troubleshoot it, so now my trail miles are untracked.