This shouldn't be as hard as people are making it out to be. Look at your total distance and elevation. Divide by number of rides. If you have Veloviewer, it's right on the summary page.

Total Rides: 364
Avg. distance: 34.5 miles
Avg. elevation: 1,418ft

If I climbed the "average" amount some have claimed here, combined with the miles I log, I would do over 1 million vertical feet a year. People seem to ignore the descent that invariably follows the climb when giving their "averages." My climb to the Forest Falls campground yesterday logged 4,600ft in 20.6 miles-- so I guess my BF-standard climbing average is 223ft/mi.