Biscut Burner,
How far do you usually go when riding? How far can you ride? If you'e comfortable going that far, go that far. screw what other's think. Assuming that this will be a typical ride, Yes there will be some there that look like poster children for the next olympics, but there will also be plenty of "the rest of us" type people there as well. You're working towards that goal, plese don't be intimidated by those that don't need to have goals yet. Just a suggestion, but if you can ride 30 miles and are comfortable in being out on the road for about 2.5 hours - Go for it.
http://www.coolrunning.com/engine/4/4_1/94.shtml Will let you know what you're burning while doing rides. While riding in groups can be fun and cut down on some of the boredom of riding to lose weight, it remains you against the weight, go out and enjoy the ride now and again.
Good luck and give a shout to say how the ride was if you do take it on...