Many people struggle to run a mile; others can knock out a 10K on a whim. A few brave souls take on "ultramarathons" — races longer than 26.2 miles (42.2 kilometers). But how far can a human run ...

Is it possible to be as fit in your forties and onwards as you were in your thirties? Perhaps even fitter? Yes it is, say fitness experts: there are plenty of people in their forties and even fifties ...