An excuse to withdraw?

[Solatium]

I hope this is in the right forum. (I've been away a little while, and now I feel like a newbie all over again.)

I submitted a story to a magazine in March. I want to withdraw it. I have three reasons:

1) The story isn't actually the same genre as the magazine (though it's close enough that, in March, I thought it was). So, it's likely to be rejected, and I don't want to bother waiting.

2) The version of the story I sent out was, despite being a fourth draft, pretty rough in the prose department. The problem's easily fixable, but it makes the story even more likely to be rejected, and I really don't want to bother waiting.

3) (the immediate reason) The magazine is known for very long response times, and another (less prestigious but much more appropriate) market that I'm looking at has a deadline coming up.

Are these legitimate reasons to do something as disreputable as withdrawing a submission? Is it too soon to do so? If I do withdraw, what excuse can I give? Or can I get away with not giving one?

(I actually have several more questions, but I'll stop before it gets ridiculous; I'm terrified of writing letters.)

[Tish Davidson]

Dear Whatever Editor You Sent the Story to:

This is to inform you that "Name of Story" by [Name of Author] is no longer available for publication.

Sincerely,

Name of Author.

There - You're Done. Withdrawing before a story is accepted isn't a problem. Withdrawing it after it has been accepted and scheduled for publication is a big no-no.

[Birol]

And the response you'll get back will probably be along the lines of:

Thank you for contacting us. We will withdraw <Story Title> from consideration.

Sincerely,
The Editor

No harm. No foul.

[Solatium]

Thanks! I'm glad to know it's not that big a deal.