So count the number of steps you take, that will give you physical and mental exercise, then measure your stride, more mental exercise and then multiply the number of steps taken times your stride, convert that into feet by dividing the inches by 12 and you then divide the feet by 5280 which will give you the miles you walked. Don't use a calculator do the math with a paper and pencil and use your brain power, more mental exercise. Then you can measure your pulse, numbers of heart beats per minute and calculate what your heart rate is over your stroll even more mental exercise, but be aware that if you are not good at math then your pulse rate will go up as you become angry or frustrated because you have forgotten your junior high math skills.
Or just buy another fit bit