The most obvious difference is that with a marathon, you train a very specific way. Oh there are a lot of degrees of training, but everyone agrees, run, and run a long run once a week. But for art there is no tried and true way to 'make it', or train for it. Seriously. You can buy books, read blogs, but really half of making it is luck. No one wants to admit that but seriously you can make the most beautiful thing ever, but unless the right person sees it at the right time, and other people are in the mood for buying, you won't sell it.
Secondly, with a marathon you have a goal, 26.2 miles. All you have to do is run that distance and voila you've made it. But with art, there is no real goal. It is nebulous, it moves around, it is elusive and can completely make a 180 on you before you know it. Some ambiguous goal like 'success' is so hard to define that really, seems pointless. Realistically you can only hope to make art, and sell what you can, if that is what you want. But the goal line will move, whereas 26.2 miles will always be just that, 26.2 miles.
You don't have to carbo load for art. Oh sure it sounds like fun, and if you are in the mood you could even justify it "I am so busy with ideas that I need to fuel up so I can create all of them". But really, we see through that. No, carbo loading is NOT necessary for making art.
No special clothes or gear are required for making art. I suppose if you are a welder, or depending on your medium you may need a few things, but most of us can get by with non-high-tech gear.
You don't lose your toenails doing your art. Unless you drop that giant coffee mug on your toe on the way to your office/workspace.
Ah ha, I do have a similarity though. Discipline. Yes you need discipline for both. If you don't dedicate the time to each endeavor, and follow through you won't get where you want to go. So there, in that instance Yes, running a marathon and making art is similar.
Feel free to stretch after a quick warm-up when you are done reading this. However, the ice bath will not be necessary.