I haven’t run a full, but am in training for a half-triathlon (next summer is the target). Have to learn to swim first haha (or at least be more like a fish and less like a stone). So, I can only speak from personal experience, but I’ve had both issues you have had (or worry about):
Blood sugar rising from exercise: you might look up information on resistant starch and Type 1 diabetes. Personally, I’ve found that 30g of RS a half hour before starting exercise completely flattens the exercise spike.
Blood sugar falling form extended exercise: mine tanks at about 45 to 60 minutes, reliably. I see it the most with cycling, which is just steadily intense enough at pace to reliably burn through muscle glycogen like mad. The solution for me has been to do a ton of BG testing while riding (no CGM, I just use finger sticks), charting my BG, and using Clif gels periodically to stabilize my BG. I’m sure it’s different for everyone, but I can handle a full gel pack (24g of carbs, 12g of sugar and 12g of starch) at 60 minutes into a ride without rising more than 10 mg/dL, if I’ve really been pushing it. I take one every 60 minutes thereafter if I’m continuing to ride. I also end up doubling or tripling my post-ride carbs in my next meal (not quite to the “eat pasta after riding/running/lifting” stage, but I’ll have 40-50g including fiber after working out). It works for me, but took a lot of testing to figure out the break even points.
What does not work for me is “carb loading.” If I try to take dextrose or any other “simple” carb before working out, my BG will absolutely skyrocket. I know it works for some people, but I really don’t want my BG at 150 or 200 mg/dL before starting exercise, since my exercise bump will send it another 50 mg/dL higher yet. I’ve found it’s best to time my carbs after I begin exercise, so that I’m digesting them as my muscles need them.
Both of the above strategies only work while very well hydrated for me, with plenty of electrolytes (magnesium and potassium, in particular). If I really want to screw up my BGs (high or low), getting dehydrated is the worst.
My biggest challenge right now is figuring out exercising in heat. I don’t know what it is, but even mild exercise will tank my blood sugar when the temperature is above 80 degrees F. Yesterday, it was 95 here, and I was working on some fencing for my yard. Feeling a bit wobbly, tested and was 60 mg/dL, down from 95 mg/dL an hour after lunch…