Really sensible post RK
half of us will become diabetic based on current experience, and weight and diet are central. Exercise immediately before the test won't help and may produce a transient increase
However there is some very recent research which looked at circulating lipids in sedentary 50 year old males and then got them do do light exercise. The lipids improved. Lipids are not blood sugar but are linked and there is some evidence regular exercise may actually reduce blood sugars and reverse diabetes. This is still early data but it is impressive.
So what with the effect of exercise on muscle mass, which determines mortality if you need major surgery or chemotherapy, there is now evidence to support regular exercise