After running in my first race, the most important lesson I learned is to run your own race. What do I mean? You know your own pace, your strengths and weaknesses. Do not compare yourself to other runners. You have no idea by looking at someone if they are faster than you or if they are running their first race.
It does not matter unless you are near the front the pack and seriously have a chance to place in the top 3 or however far they go down to win prize money. You are not competing against anyone but yourself.
I was passed by older, younger, bigger, smaller runners than me, but I stayed with my plan. I finished strong and with a personal best.