Some take 26.2

 Oh My. It can be done.  I know.  I was there.  I saw people do it with my own eyes.  They came out of the dark with bouncy head lamps, and headed back into the dark as a haloed silhouette.  Many made it.  A few did not.  Yeah, I don't have the miles.  That's been done too.  I know the guy.  A couple of them actually.  Course I know a few more that did have the miles and still had to pull up short.  When they left, I said good luck, and went back to attempting to hump my campfire in the cold predawn.  Tending the greatest beans ever.  (Not my doin).  I've done the math.  You could walk it and still finish.  That's just as hard.
Some take a few more than that.  I have to try.