Somehow I think that with the nation saved from a civil war and other violence she would have used any free time she had to raise the living standards of the impoverished people she so deeply cared for, not to train her martial arts. I'd imagine most of her time was spent learning administrative lessons, history, culture, and other things that would serve her well once she ascends. Being a friend of the Straw hats might be a permanent part of her inner character, but nevertheless she is a princess who took her responsibility to the people very seriously.