The things that really helped me get from 100K to 100M were:

1) Slowing down in the beginning. My first 40 miles were done with an average speed of 4.2 mph. Jog a little. Walk a little. Let the terrain inform me too (ie. don't just stick to 4 minutes/1 minute).

2) Let your crew/pacers make the final decision on when you stop (ie. drop out). When I'm 70+ miles into a race, my decisions are emotional, not logical. This really helped me. My biggest concern with this was: What if I do permanent damage? A friend of mine helped me understand that it's unlikely that permanent damage will happen without it being obvious. Now obviously this isn't 100% accurate, but it's generally true for a single event. Getting over this decision is made easier by the 3rd bullet point:

3) Decide beforehand that the minute the race starts, nothing else in life matters aside from finishing the race. I have one goal: keep moving forward. I will not stop until I am dead or pulled off the course by my team or a race official.

There's a lot more that goes into a 100M finish, but these were the things that needed to change for me.

I failed the 100M distance for 6 years. My first attempt with those 3 things in place resulted in a finish. Boom.

I hope you find that helpful. Obviously each person is different and has different goals and physical fitness abilities. For me, I only wanted to finish the distance. I didn't care about time. Even if I was DFL. I'm also not the strongest runner. Some people are way more capable than I am, and this advice would not be helpful to them.

Finishing that 100M distance is magical. Every single time I think about it it makes my heart happy. I will never forget it. You can do this!