You should ride at least 40 miles (45 would be better) before the ride. That's a confidence booster, and you'll see if you can tolerate sitting on the bike that long.
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Being one of the last riders to finish isn't great.
Our local event ride planned for the slowest riders to do about 10 mph. The routes were hilly though, and that really slows down the average speeds. An equivalent flat ride might be a 12 mph average?
We did accommodate one rider that was determined to finish, keeping track of where he was on the course. He finished over an hour after the rest of the riders. That's not really fair to the volunteers.
Event rides are great for riding much longer (or more elevation!) than my usual distances. The frequent rest stops with food and water are really helpful. Riding with some other riders helps the time go by quickly.
But I've been in the "last few riders on the course" group a few times. The rest stops can run out of many kinds of food, and there's at least a little personal or event support driver pressure to go a little faster, or be transported back. And the post ride meal is often just about out of food too!