You need Sheri Colberg's book, "The Diabetic Athlete." It give suggestions for adjusting insulin and food to cover specific exercise.
As a rule of thumb I drop my overnight basal to 50% and wake up at 3am to make sure I haven't dropped more. If I've worked out in afternoon or evening I also under bolus for dinner. Then just test often to see where highs and lows come up. I also under treat a high.
Usually after a week or 2 the blood sugar drop becomes less drastic.
When you workout your body takes glycogen (stored glucose) from your muscles. Then it replaces those glycogen stores with glucose, which drops your blood sugar. Over time your muscles get more efficient and require less glycogen to do the same workout, so in theory your blood sugar shouldn't drop much if you work out consistently.
But get the book. I'd had D about 25 years before I first read it and had never understood the effect of exercise on blood sugars. Even doctors couldnt' give decent advice. But the book had great suggestions and made sense.