Good news the Watch learns your stride length and calibrates itself while running with your iPhone. As most runners know (not sprinters) cadence is fairly consistent (about 180spm) while stride length determines your speed. By learning your stride/speed the Watch can better estimate your distance by calculating cadence. This info is revealed in Christy Burns blog post. http://www.apple.com/watch/christy-turlington-burns/ It says: Also it shows how easy it is to change bands.