Good points. It reminds me of two 'first' centuries for me.
The first was pretty much like your description. It was a supported ride and I jumped in here and there with different groups. The biggest thing I ran into was the support stations running out of stuff around 2/3's of the way through. Naturally it was a slow pace I was on.
Fast forward several years
The second 'first' was one where I had a goal of riding it in under 5 hours. I recruited two riders that I trained with. One was a serious Ironman participant doing multiple per year. The other was a former NFL receiver (he rode with me on his slow days mainly because I never said no.) I told them both I know I can't ride with you, but I am asking you to ride with me and they did most of the pulling, like 80-85%. After the first 20 miles, I think we were averaging about 25 mph. No one was near us. Then at about 45 - 50 miles, I said if we don't back off, I am going to blow up. We did and a big group caught us at a ferry crossing. Coming out of the ferry it climbed up from the river, I pulled out and said, no mas. One buddy stayed with me, one went with the group which I think finished at 23 mph. My other buddy and I did not make the goal of 20 mph. We came close at 19, but I can tell you those last 35 miles were the most miserable miles I have ever spent on a bike.