Go take a basic free fitness class at your local community college or YMCA. You will then understand why your edit really means nothing. There is a reason why the best athletes of almost any sport do things that they may not like to get better and build endurance.
If you move fast, i.e. get up quickly after you are thrown as uke, it should build up some stamina. Same with the warm ups before classes and weapons training, if you do it with intent and seriously, e.g. the rolls, it should help with stamina as well. I think my stamina is relatively good just by falling and getting up quickly and doing the sword katas with intent.