I ran a half-marathon last fall, and I am training to run a second.
It seems to me that training for an event like a marathon or half-marathon means figuring things out as you go along. That's what it was like for me last summer when I was training. Even after 11 years of having type 1 diabetes, I'm still figuring things out, so training is just an extension of that.
Here's what I do:
On a normal (non-running) morning, I have a small bowl of cereal (22 g carb) and 1 unit of Humalog. When I am training for a half-marathon (running between 5-13 miles after breakfast) I only take half a unit and eat a much bigger breakfast (usually a bagel with cream cheese, a banana and cottage cheese or yogurt).
My blood sugar is usually crazy high (13-15 mmol/l; 234-270 mg/dl) before I run. I don't like being that high, but if I'm any lower than, say, 12 mmol/l (216 mg/dl), I'll go low rather quickly.
I have to stop and test several times during my run (obviously, the longer I run, the more times I test). When I ran the actual half-marathon, I tested every 20 minutes.
I carry Sun-Rype fruit bars and Skittles with me to raise my sugar. I know that Skittles probably aren't the best thing to eat, but they're fast-acting and don't melt in my pocket. There are lots of specialized sugar sources for runners, but I like Skittles because they're cheap and easier to dose (1 candy is about 1 gram of sugar). Plus, I don't really like them, so I'm not tempted to eat them when I'm not running.
When I ran the half-marathon last fall, I started with really high blood sugar (16 mmol/l; 288 mg/dl), which I think was partially caused by the stress/excitement of running the actual race. It took longer to drop than it would during my training runs, and I felt a little dragged out. But, I knew that if I took insulin to lower my sugar I'd have low blood sugar, so I carried on. Once my sugar came down, I felt fine and finished in my goal time.
I think that what works for one person may not work for others, but it can be hard to find information on this topic, so it's helpful for us to share with each other. Best of luck with your training!