30 odometer miles per engine hour seems about average (to one significant digit - varies with proportion of highway vs city vs idle hours). That would suggest all your 20+ year old cars are under about 120K miles? Or they have a very high mix of freeway miles. Either way, they're not likely getting the average 10-15k miles per year.
Conversely, that rough evaluation is making me question whether my recollection of nearing 10K hours was correct - the vehicle is under 200K miles, which would suggest <20mph average.
