Since I cycle, run, walk, hike, stretch, and "gym", I use "time in heart rate zones" as the common metric among them all. (I don't care much about miles.) Recording this stuff encourages a discipline and lets me see the relative proportions of workouts so that I get enough zones 4 and 5 (for that time trial looming spring '09) and still get the "active recovery" of zones 2 and 3. Yes, there is a mild "apples and oranges" relationship between HRs derived from cycling and running. My reasoning is that there's a lot more similar than otherwise and I'm pursuing overall fitness whilst reducing boredom/burnout.
Every day, I dump (transcribe) the data* into an XLS file along with daily weight, blood pressures, and resting heart rate.
Of course, all this implies having a mid-level or higher HR monitor.
If anyone's interested, drop me a note in a personal message with your e-mail address and I'll fire you a copy of the XLS. If you're a miles recording kind of bloke, you can add that.
* - Here're the data I track:
Total workout time,
Time in each of HR zones 1 to 5,
Percent of workout time in HR zones 1 to 5,
Average HR for entire workout,
Workload Index (a calculation that allows a rough day-today comparison of effort expended regardless of workout type), and
Three-minute recovery HR (for those Zone 4 or 5 days).