Jan 1 through second weekend in June I track in Excel against a training plan to prep for the local BikeMS to be sure that I'm ready for the 150 mile weekend.

June through December I just ride my bike but do keep track via odometers on the bikes so I know what my total year's mileage is. However, I do not have computers on the mountain bike or single-speed because, to me, one of joys of those bike is not to worry about things like miles.