You can apply a sport-specific pace/speed settings in the Polar software. Go to Options → Person Properties → Sports, choose the sport and select the speed/pace measurement unit you wish you have. If you select Automatic, then speed/pace unit from Options → Person Properties → Measurement Units is used.
We recommend keeping with one measuring system, either the metric or imperial. In other words, instead of having speed as km/h and distance as miles and, please consider having speed as mph together with distance as miles.
If your exercise does not display the desired speed/pace, check in the Training Diary view that you have the correct sport selected for the exercise.