Asking for first time Marathon Advice: My longest run so far is 18 miles with just over 4 weeks to go

So I've been marathon training for a while now for my first marathon on some rather hilly routes (more hilly than the actual marathon but even that isn't flat). I was feeling quite good about this but today I intended to do 18.5 miles today but found I was running on empty at 14 and had to stop.

Am I screwed? I'm rather anxious person with no one around me who does any long distance running so this has no helped my confidence. But if anyone who does this regularly has some advice on where I am and what I should do with my remaining time I would much appreciate it.

PS. for a full marathon my 18 miles pace would have put me at 4:25hrs. I do not even remotely expect this on the day doing the full 26 miles but its for context