Me and my characters' trip through the mud was originally supposed to be an exercise in pushing out shorter content on a more rapid timetable - something around 5 pages long that I could pump out (relatively) quickly and without too much effort. Instead, it came out closer to 8 pages and took nearly as many months to complete.
The turnaround time can partially be explained by a new practice where I'm juggling between several different stories at a time, jumping from one to another when inspiration strikes or when I'm hit with writer's block. I think it helps somewhat, though spreading myself between more than two at once is probably detrimental.
As for the story itself - I usually first start from visual references of characters that I think pair well together, and then work to come up with a scene afterwards. I had independently come across both images for this story a long time ago, but it took me a few years to realize just what they had in common. After that eureka moment, the story practically wrote itself. Mud wrestling is a bit of an odd choice for me, given that I suck at writing grappling and holds, but I think it works in the end.