To take Arne Haase as a role model: Germany's oldest marathon runner is still doing 20 kilometers twice a week at the age of 90. And it doesn't have to be that sporty. But wouldn't the Haase model be a nice vision of you in your old days, being "trapped" in the body of a 45-year-old? For millions of people, regular exercise is part of their lives. But almost all of us run the wrong way. To stay mobile from now until old age, we can make up our minds for a fundamental change. E.g. we mostly...