There is a very useful book - The Diabetic Athlete's Handbook, written by a doctor who is both T1 and an athlete. I used it when I was distance rowing.
If your event will take you about 3-4 hours I wouldn't worry overmuch. Make sure you have sugary drinks or glucotabs and use them, and go for it. You should by now be running almost the full distance in training. Have you been fine then? If so, I wouldn't worry.
The only time I made special adjustments was for an ultra-marathon of 100km but my consultant took an active interest and advised me about bolus reduction and the carbs I'd need, and that I should wake up in the night after the event and eat. I chose chocolate so munched a large bar of chocolate at 2.00 a.m. and woke up in the morning with my bloods at 7.2 which was OK, if not great. I burned carbs like the clappers the next day and had to keep eating to stay level, but after that it tailed off again.
Just for a normal 26 miler though, I didn't make any adjustments except to have glucotabs on hand, and to stay hydrated.
ETA I miss those days but not enough to make me start again!