This is brilliant. Fit-to-Fat stories are fun, but often go in a very extreme direction, like having a character go from 130 pounds to 500. But this feels more realistic, balanced and even considerate.

You also make a good point, one that is rarely addressed in stories like this: it's always easier to stay in shape than it is to get in shape. Shay deciding to purposefully gain weight to share the pain of her trainees should real consideration and kindness.

Her dynamic with Yuki was really sweet too.

Overall, this is a great story.