The stride length is used to calculate distance. It is a global setting that affects the distance measurement of all days.
If you changed the stride length right now, the distance would change right now.
The stride length is not used to detect a step. Regardless of your stride length setting, the pedometer will count each and every legitimate step you take (as long as you walk for at least 7 seconds).
If you use the pedometer primarily for running, you should set the stride length to your running stride length. You should also reset the distance before your run, since all distance calculated by walking is not correct.
There are at least two ways to determine what your running stride length is:
1. Have someone watch you run and mark off ten running strides in the middle of your run. So you would run 20 steps and your partner would mark off the 5th and 14th step. Then measure the distance and divide by 10.
2. Change the stride length repeatedly until the distance equals the measured distance.
If you are using the pedometer to record both walking steps and running steps, you will not be able to get a correct distance but you will get a correct step count.