Sports
There are many sports in this world.
I can suggest one tiny tip as to Sports that you may not have considered before and that is that sometimes when exercising a change in how you view things can be useful. For example, you don't need to use "doing everything right" as a motivation, re-doing even the smallest failure, although you can do that. You can make learning a lot more fun, and if you have "never been good at Sports", this may make you good at it and enjoy it.
Swimming is a great sport for quickly raising your stamina, if you are honestly just really not used to Sports, as I wasn't, having generally sat out Sports class in 1st to 4th grade, until I decided to join when I got closer to everyone.
Jogging is a great sport for slowly raising your stamina, though you may not be a fan of slow learning, and may be a fan rather of challenge learning, or of Swimming, in terms of staming raising. Quick rises may be done with Running, although you should, if you feel as if you may burn your leg out, take a break, unless you must run, and do things more slowly, in that situation.
The best way I can put this is that you shouldn't be like a Japanese bos, overworking your leg, and not listening to it's feedback. You should be a [Japanese] leader for your body.
I have tried Aikido and Martial Arts are incredibly fun, though discipline only comes not from it, but from within, and from *happiness*, something your coach may not understand at times. If so, try to just forgive it, unless they're the annoying type, in which case you shouldn't push yourself for the sake of discipline, inner learning, or anything. Get out of all situations that are not a 10 to you in your life.