What's the weather like for this trail race? I ask because there's a product called a frio which a lot of people use for hot weather to keep their insulin cool.
Are you on injections, or a pump? You'll want to have syringes either way, for backup if you're on a pump.
If you have a pump, spare batteries, infusion sites...
I just realized I'm assuming you're a type 1 on insulin.
Anyway, it's pretty much common sense. Think of what could go wrong, and prepare for it. Having had diabetes for decades, I have an idea what could go wrong, having had it go wrong a few times.
I would keep your low bg treatment food, glucose tablets or whatever, separate from your regular food, so you remember to conserve it so you have it if you need. it.