Average speed doesn't mean much unless all of your riding is either flat or on a given course.
For instance, I could do roughly the same ride, except for which road I take, to the river and back. One route has 500 feet of climbing, the other has 2,000 feet. It's more work to average 12 mph on the second route than 15 mph on the first.
That said, they're both 32-34 miles. I track miles because it's simpler. You could track any number of things -- speed, distance, climbs, average or maximum heart rate, etc., etc. But make sure you're having fun, or you won't want to ride.