kids who walk or cycle to school are fitter

At one level this one comes under the heading of "obvious"!

On another level it is worth reminding the world that everyday exercise - walking, cycling etc - has a substantial effect.

Many people would do well simply to walk more like Nassim. We are meant to walk. As Kurt says:

7) Walking is an excellent evolutionary activity that we are, simply, evolved to do and doing it is nothing but good. I agree with Sisson on this - lots of low intensity rythmic activity can be good.

Anyway here is the study abstract

Aerobic fitness and mode of travel to school in english schoolchildren.

CONCLUSIONS:: These data confirm findings from countries where cycling is very common in showing that cycling to school is positively associated with aerobic fitness. This study adds to the existing literature by showing that walkers and cyclists are more likely to be classified as fit using recognized cut points. Those who walk and cycle to school may, therefore, be at reduced risk for developing chronic diseases in adulthood.