How to run 5-6 miles when you usually run 4?

Can't open the link to your background information but I gather you can run 4 miles and are concerned about running 5-6 with a group. First I wouldn't be so concerned with the group. Even the best of them can have a bad day and not keep up at a given pace or for a given distance. Also you are not training them, you are training you. If you can run 4 miles, you can run 5. However let's look at the worst case scenario, they decide to run 6 miles and you can't run that far. Come up with a workable plan for that. For example, let's say they are running 3 miles in one direction and then they turn around and run 3 back. OK, run with them for 2.5 miles and wait for them to come back and then return to the starting point with them or run 2.75 and wait for them to come back. Look for a solution that lets you run a distance you are comfortable with physically and you will gain the confidence to do longer runs mentally. Good luck!