“Running a marathon” usually means running in a marathon, not running the marathon. Jordan Brookes‏  joked on Twitter on May 12, 2014:
 
“I’d like to run a marathon but it seems like such hard work, getting the roads closed, providing water for everyone etc.”
   
       
Twitter
Jordan Brookes‏
@jordbrookes
I’d like to run a marathon but it seems like such hard work, getting the roads closed, providing water for everyone etc
7:29 PM - 12 May 2014
 
Twitter
Running Sisters‏
@RunSisWindsor
“I was thinking of running a marathon, but it might be too difficult getting all the roads closed & water for everyone.” c/o @Jordbrookes
7:11 AM - 5 Aug 2017
 
Twitter
timespliters‏
@timespliters
I was thinking of running a marathon but I think it might be difficult getting all the roads closed and providing enough water for everone
9:13 AM - 5 Aug 2017
 
The Scotsman (Scotland)
50 of the best jokes and one-liners from the Edinburgh Fringe Sara Pascoe is appearing at this year’s Fringe and has a number of entries in our top 50.
Published: 13:19 Friday 04 August 2017
(...)
“I was thinking of running a marathon, but I think it might be too difficult getting all the roads closed and providing enough water for everyone.” Jordan Brookes (2016)
     
Meow Gag
20 of the Best Jokes and One-Liners from the Edinburgh Festival Fringe
November 7, 2017
(...)
12. “I was thinking of running a marathon, but I think it might be too difficult getting all the roads closed and providing enough water for everyone.” Jordan Brookes (2016)
 
Twitter
Dad Jokes‏
@Dadsaysjokes
I was thinking of running a marathon, but I think it might be too difficult getting all the roads closed and providing enough water for everyone.
2:06 AM - 23 Nov 2017