How can I walk of the fat but maintain my butt size?

I want to walk for an hour a day but I fear that my butt will shrink. So how can I walk but maintain my butt size at the same time? I need at home workouts. I live in a apartment so stairs aren't a option. I also don't have equipment like dungbells etc. are lunges ok? What are others?

I saw another question like this but it is not me so please don't say I'm posting this twice.
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How can I walk of the fat but maintain my butt size?
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