That's easy stuff. I'm fine with cold showers, physical challenges/endurance/training... I do that for fun to procrastinate within the constraints of my aging body.
Mental discipline is more about prioritizing time with a bigger picture in mind - enduring boring mental tasks at the expense of mental stimulation/passion.
Edit: I'm sure some would tell me to meditate more - and I probably should.
I'm saying that..... I like going outside in a tshirt at -10 to -25 and seeing how long I can think of something I want to do next.... before I just have to get in... I've lasted 22 minutes at -25 with serious wind in a tshirt... there's something to physical that affects mental
It's not that cold near me (and honestly I'm finding that hard to believe cause it sounds superhuman).
I agree physical affects mental and I enjoy the interplay, but I don't think it's the whole story for me.
when it rains, go for a long walk in the rain without any protection from the weather. Listen to music and just walk until you either come back home or it stops. Pick familiar music.
edit: I've done this 2 months after a woman was attacked and lost her leg to a coyote on our road. Peace can beget peace if it only costs an old lady a leg lol