Some people training for marathons have asked me: Should I even bother with a 5K? They think this because 3.11 miles is a small amount in the grand scheme of training for a 26.2 mile marathon. My response to these people is YES, it is 100% worth it!
Running is a VERY mental sport to participate in. Being able to endure a long distance like 26.2 miles is something that is not done with muscle alone. Most runners hit a mental block at around mile 20 where they become unsure if they can keep going.
That is why I say participating in 5K's are very much so "worth it." By entering organized races during your training, even when they are short in distance, you are training yourself to enter that mental state that you will need for your future marathon. By training yourself over and over, you will find that you are easily able to put yourself in the mindset to endure and be a successful runner. The mindset will help you when you reach that 20th mile of the marathon. Since you have been in so many organized races already, being in an organized race environment is nothing new for you. You feel very much at comfort in this setting.
Now that you have run many organized races, you feel comfortable on race day. For your first marathon there will be many things running through your mind. By taking environment, you will have one less thing to worry about. You can focus solely on the physical aspect of the day and take any mental roadblocks out of play.