On the few event ride 100 mile days that I've done, I stopped at every rest stop, about 15 miles apart. Food at every stop too.
I've noticed that even stopping for 30 seconds at an overlook on a long climb kind of resets my attitude. it's like a boost in energy and mood.
Don't try to keep up with your group riders if the pace is too high. I like riding along with riders I don't know on these event rides. I try to evaluate how erratic the riders are, though, and keep a distance from those people.
It seems that every time I do 100 miles, I ride for a "long time" and then check the distance on my GPS. 22 miles done! 79 to go! What! How discouraging. But once I get to about 50 miles or so, things settle down and the finish seems doable.