Have you considered a pedometer instead? (Doesn't give you the linear distance you've walked, gives you the number of steps you've taken instead. Which, when multiplied by your average stride -- which you can get by doing a measured kilometer to average it out -- tells you the distance. The advantage being that they also give you a feel for how far you've gone on days when you're not going terribly far in a straight line but still seem to be moving around a lot.)