I have created a small exercise you can practice with to improve your movement in table tennis, also to stop yourself stretching your arm when you are playing your forehand and backhand.
I have seen many players stretching their forearm when they are trying to their forehand and backhand. By holding your shoulder with your non playing hand can significantly force yourself to stop stretching your forearm while you are too lazy to move your legs. Players may feel uncomfortable at the beginning, due to the imbalance of the body. Try to start off with a slower pace to get used to the feeling then increase the pace eventually you will start feeling how your legs are forced to move quicker because of the limited stretch from the arm.
Hope the exercise helps ! And please do not hesitate to contact me if you have any questions about the exercise.