Whether you format carefully now or later, make sure that you set up your file(s) so that you will be able to easily update the formatting later when you need to. Whatever app you're using -- Word, Scrivener, something else -- use the tools it provides to set your formatting globally so if, for instance, you need to change how your paragraphs are indented and spaced you will be able to make make the change once and not ten thousand times. In Word, this means using a stylesheet, not manual formatting. Templates are also a good idea -- set up one in your favorite tool and use it for all your files so the formatting is consistent.

But I'm a bit confused. If you're going to be writing your first draft longhand, why are you even thinking about formatting at all? As long as your manuscript is a longhand draft on paper, formatting is irrelevant. Don't worry about it until the time comes to transfer your work into something electronic.