Is it a bad thing to fantasize about running a marathon?

No, that's not a bad thing.

One may fantasize about whatever they choose. There are no mind police. It can be potentially "bad" if one fantasizes about things that are criminally illegal, but even that depends on what those fantasies are and how often one has them. 

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Your question belongs in Entertainment & Music>Polls & Surveys (as your question is a poll and not related to the sport of running, specifically) as it's currently in Sports>Running and not really applicable to this subcategory (and thus decreases your chances of getting many useful answers).

Moving your question can be done by clicking Edit (pencil icon for mobile)>Edit category just beneath your question, then selecting the category that's appropriate for the topic

Don't fantasize about it. Do it! It will be an accomplishment that you will never forget or regret. And always be something you will able to brag about and everyone will admire you about it..

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Hello Seth . . .

( Could equal ) c = perfect to day dream.

Could lead to the real thingumeeyjig.

A bottle of water c = a good idea.

Hope you have a happy Friday.

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