First Race Hydration/Logistics?

Okay i’m a little confused with how many carbs/sodium I need to be taking as I run. I’m currently about little over halfway through my training (marathon is on 08/30). I’ve started my longer runs (12miles+), and I haven’t been able to run consecutively without stopping.

I know it’s good to stop and all, but in all honesty it is striking my ego in a way. It’s also incredibly hot in the Southern part of the US, and after moving here from the north a month ago my mile lowered about 2 mins because of how hot and humid it is. Because of this, on my long runs it feels like i’m dying even tho I tend to take sips of water from the fountains along the way + 2 gels for anything above 12 miles + hydration chews. The issue here is I don’t know when I should be taking each. I honestly just take them whenever, I don’t really know how to tell when I need them.

Don’t know if this makes any sense, but I basically just need help knowing when I should start taking gels/and how many carbs I should be taking as I run. TIA