Not every “marathon” involves running 26.2 miles. “I do marathons (on Netflix)” is a jocular one-line saying that has been printed on many images.
 
“Darling son just told me they just put all the Twilight Zones on Netflix! Why yes, I do marathons! Just not the 26K kind” was posted on Twitter on April 23, 2011. “I like to do marathons….of tv shows on Netflix” was posted on Twitter on December 22, 2012. “I do marathons, On Netflix” was posted on Twitter on December 6, 2013, when the “marathon” pun went viral.
   
   
Wiktionary: marathon   
Noun
marathon (plural marathons)
1. A 42.195 kilometre (26 mile 385 yard) road race.
2. (figuratively, by extension) Any extended or sustained activity.
Verb
marathon (third-person singular simple present marathons, present participle marathoning, simple past and past participle marathoned)
1. To run a marathon.
2. (informal, transitive) To watch or read a large number of instalments of (a film, book, TV series, etc.) in one sitting.
 
Wikipedia: Netflix
Netflix /nɛtflɪks/, stylized as NETFLIX, is an American entertainment company founded by Reed Hastings and Marc Randolph on August 29, 1997, in Scotts Valley, California. It specializes in and provides streaming media and video-on-demand online and DVD by mail. In 2013, Netflix expanded into film and television production as well as online distribution. It is headquartered in Los Gatos, California.
     
Twitter
Without the Blonde‏
@withouttheblond
Darling son just told me they just put all the Twilight Zones on Netflix! Why yes, I do marathons! Just not the 26K kind. #fb
10:56 PM - 23 Apr 2011 from Long Beach, CA
 
Twitter
Briana ☀️‏
@bbbrrriiaannaaa
I like to do marathons….of tv shows on Netflix.
7:52 PM - 22 Dec 2012
 
Twitter
Drunk Cat Ladies™‏
@druunkcatladies
Do I do marathons? Actually, yes I do. All the time… Thanks to Netflix.
9:58 AM - 28 Mar 2013
 
Twitter
KiKi‏
@Neea_Kiiski
I do marathons… Netflix marathons #DoesThatCount
9:54 PM - 28 Jun 2013
     
Twitter
Sam I Am‏
@samiism
Replying to @mindykaling
@mindykaling I use “marathon”. Then I can tell my friends I do marathons all the time, sponsored by Netflix.
12:57 AM - 15 Aug 2013
   
Twitter
sierra leona 🌸‏
@sssnail_
I do marathons (of netflix)
11:08 PM - 6 Dec 2013
 
Twitter
Heather Valentine‏
@hvalz
“I do marathons, On Netflix”
11:30 PM - 6 Dec 2013
   
Twitter
Toni‏
@ToniScrignoli
I do marathons…. On Netflix #hahaha 🎥❤️
12:21 PM - 7 Dec 2013
   
New York (NY) Times
A Restorative Triumph After the Bomb Blasts
By JERÉ LONGMAN APRIL 21, 2014
BOSTON — The white T-shirts said 4.15 Strong on the front and Survivor on the back.
(...)
There were the usual funny signs on the course: “Toenails are for Sissies,” “I do marathons on Netflix” and “The Kenyans are drinking all the beer.”
(At the Boston Marathon.—ed.)
   
Facebook
Esther the Wonder Pig is with Michele Marongiu.
March 28, 2016 ·
I do marathons…...on Netflix.
 
Twitter
SndaCity‏
@SndaCity
“I do marathons…......on Netflix!” #sundayfunday #rompers and #wine 🤓 @ New York, New York https://www.instagram.com/p/BfEmglwlNmc/
3:43 PM - 11 Feb 2018 from Manhattan, NY