It's also well known that as you become fitter, your body becomes more efficient at performing the same activities. It doesn't surprise me that people who habit

It's also well known that as you become fitter, your body becomes more efficient at performing the same activities. It doesn't surprise me that people who habitually run get fitter, burn fewer calories per mile, and need to run farther to maintain the same burn rate. That these people are also getting older at the same time might just be a coincidence.