chain wear is likely more a function of chain load rather than mileage. If you rarely sprint or stand up, are fairly light, spin on well lubed chain, your drive drain will last longer than putting racing loads on it.

I've seen Machka's web site, and she does long touring rides with insane mileage (slower than human mileage tours), and I accuse manitoba of being flat as a pancake.

As obvious as it seems, expected mileage out of a chain and cassette should be a function of pedal pressure squared * mileage... instead of just mileage.