Cranking, my heart goes out to you. Might the stress be from the fact that when you do go on a group ride like this that you feel you must ride at your best? In other words, maybe you feel that you no longer have a choice in the matter, the decision has been taken out of your hands? I know there are days when I just want to cruise and enjoy the ride and then maybe push myself when and if the spirit moves me. Being something of an introvert, trying to make a good showing with all those other people around might be very stressful for me.