I was a marathon runner for 25 years before being diagnosed Type 1 last year - a week before I was due to run the Stockholm marathon! Since then I've been trying to get back to fitness and also getting used to balancing my blood sugars whilst exercising. It's a tricky business, and there is a lot of trial and error involved. Basically, exercise will make the body use insulin more efficiently, so you'll probably need less with your meals (or more food) if you're planning to exercise. The 'efficiency' can last for some time afterwards too - even into the following day, as your body replenishes its stores of glycogen in the cells and the liver.
Conversely, if you stop regular exercise for a few days, your insulin sensitivity will go back to those pre-exercise levels, so your BG will be at higher levels. An added complication is the type and intensity of exercise - high intensity can cause the release of stress hormones that will increase BG levels even though you are exercising - they may quickly fall later so you need to watch out for hypos. As you get used to a certain level of exercise, your body's energy requirements will be lower, so you won't 'use up' the glucose in your blood as quickly. As an example, the first few times I run 5 miles after being used to running only 3, my body will have fairly high requirements and my BG will likely dip. However, after a while 5 miles will be well within my capabilities so my BG will hardly change from start to finish.
Unfortunately, there are no hard and fast rules and everyone is different to some degree. I'd recommend 'The Diabetic Athlete' for a good explanation of the science behind it all.
