She already had the Stamina to Spiderman cling to a corner of the ceiling in one of the first pages. I don't think she ever needed the pool sessions.
Also, doting granny is the one to keep her secret of course.
not to mention that she also so far managed to outrun quite a lot of fairly talented umas, gaining enough distance to them so she had the time to hide, so not only does she have already amazing stamina but also speed...