Considering switching from the full to a half.

I’m a beginner runner and I don’t have any goals but to finish, I’ve been running most of this year. I have a marathon next month but I just did my 14 mile long run and it was so hard. I’m really considering switching to the half and then doing the full marathon next year. I guess I just want opinions on what you would do because I did just finish that 14 miles so maybe it’s not the best time to make the choice but to be honest a full marathon seems impossible right now.