the beginning of mass is the most dangerous part. I am always on the alert in the beginning. I have one clip out and in the lowest gear. Some will go to the front and sprint to get away from the masses. As the mob start moving there will be allot of passing and being passed. Just use common sense and realize that you might not get your normal speed for the first mile or two. After that it is pretty much like your bike path.
I often chat up a few riders along the way and just enjoy the day. Stops at rest stations are fun if you have never been before. Fruit gel pack water and electrolyte drinks as well as other snacks are provided. Stop and enjoy hey its your first time.
first and last rule be safe and have fun