One thing I'd add, that's pretty fundamental is rest. Usually on a long ride I'll be asleep when I hit the pillow, but if you get too excited the night before, wind up sleeping in a cheap motel surrounded by a wedding party, things can get dicey. A more specific issue can be sleep apnea which can sometimes assure drowsiness even on a good day. Important to deal with even if you're not touring, but even more important if you need to stay awake in critical situations.
Another issue in group rides is assertiveness. You're riding along, fatigue is kicking in, and the pep talks aren't working so well, but you assume everyone else is fine, so you grit your teeth and try to keep going. You're expecting the leader to pull over any time for a break....and they don't. Don't be timid-there's no shame in needing a break. It's worth inconveniencing the rest of the riders if you need to be safe...and odds are someone else might be grateful.
Here's Ryan, in his usual fine form:
