Run a marathon
Here are some things that they don't tell you about running a marathon. Of course by 'you' here I mean 'me', because the first thing they don't tell you is that your experience of running a marathon shares the pain, the course, and little otherwise, with anyone else's.
At big events there will be so many runners that the starting line won't just be something you cross but something you stand in. As in, there will be a huge queue to start the race. The first like ten minutes of the race will actually be enforced walking, you'll be so packed together.
Heads up: chafing's going to happen no matter how well you try to avoid it. Here are a list of the places that it can happen: between thighs, between thighs and testicles, your armpits, your nipples, your perineum, somehow. I don't know what sort of karmic influence reigns over this kind of thing but even if you yank your pants up your ass and bandaid your chest and vaseline your armpits you are still going to end up with fields of inflamed skin that will burn in your post-race shower.
You will get to know everyone's backs very well, and when you pass someone it will be a big deal. When you get passed you'll force yourself not to notice. Which leads to a very strange situation where you'll pass the same guy like eight or nine times without ever noticing him passing you, and you'll start wondering if you're hallucinating or something, conjuring extra racers to pass, like your brain is doing you a weird, weird favor.
This is actually complicated by the fact that there are groups of people who purposefully dress exactly the same and so the guy in the purple shirt with 'Set A Record' across the back, who you've passed now it feels like twenty times--you've actually only passed him maybe ten times, and the other ten times were his buddy who is identical except for his shoes. Or was it?
In fact, an uncanny number of people will be wearing these 'Set A Record' shirts, from a variety of brands. Uncannier still is that the number of these shirts increases dramatically towards the back of the pack, where no one is going to be setting any records, except maybe personal ones.
On the topic of uncanny gear: you might pass a trio decked out in bike gear--including padded shorts--upon whose backs is written 'WE ARE ALL ONE'. At the beginning of the race, in the huge pre-starting-line peloton, you'll probably be like, "Yeah, tell me about it." Later in the race, though, when you see them again, your eyes will be wet and unfocused with pain and exhaustion, and you'll swear that their shirts actually say 'WE ARE ALONE', and you'll probably still be like, "Yeah, tell me about it."
After running for a while on even pavement, even the slight topological anomalies of a semi-kempt lawn will feel like running on sand. The planes of your foot will get all fucked up and when you return to asphalt you'll have a hard time not getting down on your knees and humbling yourself before the long ribbon of tough gray stone.
You will feel like everyone else's calf muscles are bigger than yours, even the 80-something, totally hairless dude who you have passed seven or eight times. And especially the buzz-cut guy with the huge shoulders and 2% body fat, whose calf muscles have more striations than you have muscle groups in your whole body, and don't you forget it. He will power past you early in the race and you will not see him again.
You might run for a while near a lady whose calf muscles are so big that she actually has calf-muscle stretchmarks, which you'll probably ogle for a while before losing her at an aid station, where you'll be too busy ogling brown, semi-slimy bananas to notice where she went.
If you have any old injuries that you didn't set correctly, or allow to heal right, or anything that isn't lined up just right in your body, it will start to act up between kilometers 20 and 30. Old rotator cuff strain, broken nose, maybe you had diarrhea really bad once--it will come back and nibble on your nerve endings until you are done.
It actually takes a lot less time for the pain to start than you might think. You can go from feeling dominant and great to feeling like the limp green stuff that remains after mowing a lawn over the course of like two kilometers.
Communion feels great. You will hit the wall--it is not a myth--in fact it is Real with a capital R--and you'll wind up walking kilometer after kilometer, watching the pace on your GPS watch increase and increase, and the choices and the pain and the weariness, the limp, moaning thing on your shoulder whispering Just walk, the choice of what stupid music to put on next, the fear of the rapidly clouding sky, will all have been multiplying in intensity and complexity, and by this point it will be all but impossible to start putting up the clicks at as steady a pace as you were when you started. But then a pacer, who has a balloon on his head and a sign on his chest indicating how fast he'll take you to the end if only you keep up with him, pulls up behind you, and looks you in the eye, and gives this totally effortless smile, and touches your shoulder, the same one which has been giving you this terrible tweaking pain since kilometer 31, and says, "Come on, run with us." The crowd behind you will bob and sweat and breathe, and their footsteps will patter for all the world like a huge tortured forty-leg animal. But the hurt, the confusion, the worry, the dread, the mounting despair, will all fall away, and it will be the easiest thing in the world to nod, smile, say "Yes," (in English for some reason), and start running again. The pacer and the huge animal behind him, will take you right to the doorstep of victory, but no one can carry you from the point where you see the goal onwards but yourself.
This is where my experience, if it shares anything with yours, will almost definitely diverge, unless you were running the 2015 Okhotsk Marathon in Abashiri, Japan, and finished around the 5:30 mark. It is unlikely that as you round the final corner, the rain will sweep in in torrents, that it will batter a huge field of sunflowers. It is unlikely that lightning will come down across your vision, that the thunder will roll through your fragile body. It is unlikely that teenagers will reach their long, wet fingers through the storm to high-five you as you go. You will probably not taste 42.195 km of accumulated salt on your forehead drip down across your lips, into your eyes, although I guess it's possible. It's also possible, but not so likely, that the world will blur into a gray-and-yellow whorl around a large inflatable arch, and that you will hobble at speed through this arch like a suicide towards the light.