My understanding is that, for lower levels of exercise, such as walking, the body is in an 'aerobic' state, and glucose is used normally, with a consequent drop in BG levels. When exercising hard, the body goes into an 'anaerobic' state, and glucose is used more rapidly, so the liver secretes some of its stored glucose to make up for this. In the diabetic body, the extra glucose stays in the blood. For me, Type 1, I have to take extra insulin to bring the BG back down; not sure how it works for you. The glucose secreted by the liver is later replenished from food, so I often get hypos later on and/or require less insulin. Again, not sure how this'll pan out for you...
Hope this helps.