Based on the 11 hour 5 min match at WimbledonfeaturingNicholas Mahut (FRA) and John Isner (USA) where they played 138 games in the fifth set alone and where most records were completely shattered.
It refers to anything an event or oddity that will almost positively never happen again.
An exceptionally long episode of masturbation fits. These episodes are usually brought on by sexual deprivation and bordom; however, and I'm not making this up, they have been used a means of fundraising for sexual related causes such as safe sex. A masturbation marathon is a strenuous event that requires disciplined training and preparation. There are several related subcatagories such as time, ejaculation distance, and most orgasms for both male and female competitors (I'm totally serious - google it).
Something of a daily occurance, NCIS is a great show and it seams its so great that they have "Marathon's" almost everyday. Do not get sucked in as these last for 12-24 hours and you will get nothing done.
Person 1: I'm going to work on my project *walks into TV room* EWWW!!! NCIS Marathon! *Two hours later*: Shit I got nothing done!