Walking won't really do anything for your stomach in terms of toning no, it's more toning of the legs - unless you're pushing a buggy up and down hills.
I think it depends on your overall body mass index in general.
For example, I'm 5ft 6 and around 9st 9lbs which isn't big, my diet isn't bringing my stomach in, although I do eat generally well, because its wrinkly and just changed shape from being pregnant.
To bring it in I need to do stomach focused exercises because it aims directly at that area to get it more defined.
If I was overweight then yes a diet would help bring my belly in to begin with, but again exercise is what needs to happen.
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