Hi Jeff, and all-- my first post here! I'm in my 50s and was just diagnosed T1D a few weeks ago ... My way of dealing with the overwhelmingness of it is to do a lot of searching the internet! Especially for good academic articles.
I've never run a marathon but have always done a lot of brisk walking, and took up gentle jogging via 'Couch to 5k' a year and a half ago. After diagnosis, and starting to find out about managing T1D, it soon became clear that even this level of exercise had to be taken into account.
Then I came across this:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7702152/pdf/PEDI-21-1375.pdf . "Glucose management for exercise using continuous glucose monitoring (CGM) and intermittently scanned CGM (isCGM) systems in type 1 diabetes: position statement of the European Association for the Study of Diabetes (EASD) and of the International Society for Pediatric and Adolescent Diabetes (ISPAD) endorsed by JDRF and supported by the American Diabetes Association (ADA)"-- sounds daunting, but very helpful tables!
For the purpose of these tables, you would probably count as "Ex 2", "intensively exercising". Assuming you have a Libre or other 'flash monitor' or CGM, Table 1 tells you what to do just before exercise, depending on what your monitor is saying; Table 2 tells you what to do during exercise, depending on what your monitor is saying; and Table 3 tells you what to do post-exercise, including overnight if you were exercising in the late afternoon or evening.
It looks like you would want to start any long run with glucose around 10, maybe even higher, and set your low-glucose alarm at around 7. Then, if/when your alarm goes off: if the trend arrow is up, fine; if it's level, take 10g carbs; if it's angled down, 15g; if it's straight down, 20g.
No doubt there will still be trial and error on top! I love the way trophywench puts it, "Personal Clinical Trials using our own bodies as Crash Test Dummies." But at least the EASD guidelines give a good solid research-based starting point.
I haven't tested 'em yet myself-- lost so much weight before diagnosis that BMI went from 18.5 to 16, feeling pretty washed out/up, so taking it easy until I get some weight back-- but would be very keen to hear others' experiences!
Finally, about insulin: muscles have an insulin-independent pathway for using blood glucose. So, prior to a long run, you would want to inject less insulin. (See for example
http://www.runsweet.com/diabetes-and-sport/ )
Good luck, and let us know how you get on!