Personally, years and years of writing perhaps 100 words per year that aren’t my signature has made my handwriting slow, even uglier and harder to read than it already was, and has probably (I wouldn’t know) ruined my stamina for handwriting. I’d have to start such a letter-writing project with exercises to restore my handwriting to some minimally acceptable level, and my experience has been that adding stuff-to-do-before-I-do-a-thing usually means the thing won’t happen.