Died at mile 17 during training 4 weeks out. Is this indicative of not being prepared for the marathon?

Race is 14MAR. Just did a 20 miler and the first 10 miles felt very solid, miles 11-17 were tougher but still ok, but at mile 17 things went downhill fast.

I hit a wall hard and could barely shuffle my feet in front of me. My pace slowed by a whole minute.

I’m feeling a bit discouraged and nervous that this will happen to me on race day. Is this indicative that I am not where I need to be training wise? Admittedly, I probably did not fuel adequately yesterday and the day before in preparation for this run, so that could be a factor.

But I’ve heard the 20 miler is more about confidence than fitness, and honestly it kind of took a blow to my confidence instead of building it up.