One of the biggest challenges I face as a type 1 diabetic is accounting for exercise. I'm not actually terribly active, but I do own a horse and try to ride him several times a week. Even some of the lesser stuff, such as grooming or mucking, can be surprisingly good exercise.
Exercise is challenging for insulin-dependant diabetics because of the risk of hypoglycemia. I have had instances where I am riding my horse or cleaning his stall, and suddenly find myself dizzy and shaky. And sometimes it has even happened later in the day, hours after I've returned from the barn.
Exercise brings on hypoglycemia because of the fact that the insulin is already in my body. Most diabetics take some sort of long-acting insulin that is design to hold their sugars steady over an extended period of time. (The exception is the insulin pump, which delivers short-acting insulin at a steady rate the the diabetic can change as needed — or even stop altogether.)
When a person exercises, his or her body suddenly gets several times more efficient at using insulin. A non-diabetic person's pancreas slows insulin production by about 80 percent while he or she exercises, in order to compensate. But since I take a shot of 24-hour insulin (called Lantus) every evening, I'm stuck with that amount of insulin.
In order to prevent myself from crashing, I need to remember to always eat a snack before I exercise. I've taken to running my blood glucose slightly high before I head to the stables, since I never really know how much the exercise is going to affect me. I can always check my blood sugar again and correct as needed when I return home!