Black shorts are your friend
It's entirely possible that at some point during your training you may have an "accident." Blame having children or those pesky digestive issues—either way it may happen. The important thing to remember, in this situation, is that almost every single runner can tell you about a bathroom incident that happened to them—from peeing just a little with every step, to full-on “there was no bathroom in sight and it just happened” stories. All you can do is remind yourself that it happens.
Running may be freeing but it's most certainly not free. From running specific shoes to the fuel you need on long runs, to the inevitable increase of treat-yo-self moments, the price of training for a marathon can add up quickly. Enter the marathon with a budget in mind. Do you really need three gourmet cupcakes after a long run? Figure out where you can save and where you plan to spend, this will prevent any mid-training cycle freak out moments where you feel like you can't afford to be a marathoner.
Your feet may hurt—bad
You went to your local run specialty store and found the shoe that the sales associate said was perfect for your gait and arch height. You sized up even though you thought it was crazy that your regular shoes and your running shoes would be a size different. You liked the shoes at first, some days even loved them, until the day you ran 20 miles and felt like your feet had been run through a meat tenderizer.
Twenty miles is a long way to go. It's completely normal to have shoes with plenty of cushioning that are great for your biomechanical needs and have them feel terrible after your long runs. Don't fret, this terrible feeling eventually goes away as you build up your tolerance to longer distances. Ice, roll out your feet, or take an Epsom salt bath as needed post run.
You wake up in a cold sweat thinking that you just somehow missed the start of your race, only to realize your race isn't for another month or so and tomorrow is a rest day. Worrying about what could go wrong can affect how you sleep. Combine that worry with restless legs after a long run or an overall feeling of exhaustion and it can be a recipe for some pretty crazy running nightmares. You aren't losing your mind, most of us have had a crazy running dream at some point.
Everything can go wrong during 26.2 miles
You planned for this day. You practiced your fueling strategy, eliminated suspect foods, wore nothing new, got plenty of sleep, drank an appropriate amount of water beforehand— not too much, not too little—you remained calm and didn't start too fast, yet everything went wrong.
You may feel like a failure but you aren't. Any time your body can carry you 26.2 miles, whether those are bliss-filled miles or miles where you struggled, is a victory. You may have done nothing specifically wrong, it just wasn't your day. Use it as a reason to sign up for another 26.2. Everyone deserves one good race before deciding that marathoning isn't for them.
Life goes on
You, the person who didn't believe 26.2 miles was possible, just ran that far. Congrats! You may think that everyone else cares, and maybe they do for a day or possibly two, beyond that, however, their life goes on and not a thought will be given to your race. You, on the other hand, can't stop talking about what happened—detailing how you felt every mile.
Seek out someone who has run a marathon before to tell your stories to. They are more likely to listen and relate and likely won't brush over your accomplishment. Also remember, at the end of the day, what others do or do not say to you after your race doesn't matter.
Find your next race.