Last night was a fun 47 mile bicycle ride at an average of 19.8 mph with a dozen of my friends. I was gassed from mile 15.
Bicycle riding, even at speeds of 20 mph is very aerobic. I digress, let me clarify, the transition from aerobic to anaerobic is much faster and harsher on a bicycle. In other words, grinding up a hill standing up or on a 30 mph+ sprint my legs get tired real quick, as in a few minutes or even less than a minute. However, sitting down I can ride at 20 mph for at least an hour, even on my own sometimes (once). The point being, often times the group pushes harder than one might desire, and each person has the choice of sticking with them or falling back. One of the benefits of group training is that some days when people are feeling mediocre they are pushed to work a little harder. Most people can stand to work a little harder.
In short, just hang on. Especially if it is aerobic, like a bunch of college kids that take a recovery day and turn the last 5k into a 6 minute mile pace light tempo. It's not the workouts that knock your fitness out of the park three times a year that make any one person into a world beater, it's the consistent week in and week out minute improvement year over year that make a difference. Some days are going to be hard. Not only days running, but days working wherever you work, or relationships with people will be challenging, so just hang on. Consistency, or hanging in there when it isn't comfortable, in just about everything is regularly what separates the successful from the unsuccessful.