These tips are entirely based on my observations while running the 2019 NYC Marathon with Syd, his 11th time running the race. He is not a professional runner, but has a pretty good track record at finishing marathons, and is generally a ray of sunshine.
In the Months Before the Marathon
Train by running a lot.
In the Hours Before the Marathon
Make sure you go No. 2. Going No. 2 before the race will shave anywhere from two minutes to eight minutes off your marathon time by preventing you from having to stop mid-run at a port-a-potty to go No. 2, and potentially wait in line for several minutes at said port-a-potty.
Bring enough snacks to eat so you’re not hungry when you start running.
While Waiting for the Marathon to Start
Wear a black trash bag with a hole cut for your head. This will keep you warm, as well as give you privacy if you need to apply Body Glide to sensitive regions and/or take a last-minute pee in an emptied Gatorade bottle (which you have also brought along).
After the Marathon Starts
Run 26.2 miles without quitting.
Cheer back at spectators who yell words of encouragement in your direction, with the goal of pumping them up as much as they pump you up, if not more.
Stop, and even run backwards, to help other runners up if they trip and fall.
Avoid pooping your pants at all costs.
No matter how tired you are, summon the energy for dozens of genuine smiles.
Hug your wife, mother, and father, and anyone else who has come out to cheer specifically for you along the race course.
Every once in a while, yell YEAH at no one or everyone.
Thank all aid station volunteers.
High-five all signs that say “Push here for power boost” or similar.
Generally focus on being grateful you can run 26.2 miles and if the weather is nice, be grateful for that, too.
When encountering race photographers, give no indication that you have been running for one, two, or three hours, and give them a pose and smile worthy of printing in next year’s marathon marketing materials and/or signage.
Pump one or more fists when running past live bands and DJs.
Suppress all complaints and other negative verbal statements until the end of the race.
Run your fastest two miles from Mile 24.2 to the finish line.
At the Finish Line of the Marathon
Congratulate people around you even if they are complete strangers you have never seen before and will never see again.
After the Race
Use pizza to replace all the calories you burned while running, and use additional pizza as necessary.
Talk about maybe not running a marathon next year, but leave it open so you can maybe talk yourself into running another marathon next year.