Because of a lungdefect i can't build up long-term stamina. I have only 2,7 liters lungcapacity (a 'healthy' person had about 5) in other words: running is no option so i must build up all my ring stamina in a different way.

I train 3 to 4 times a week, and what i do, which greatly helped to get me in ringshape, is:
At the gym I jog 5 minutes, take about half a minute rest, and jog another 5. If i'm late because of work and the gym is too full of people, I do 2x5 rope jumping. This is just to get my blood pumping and my body ready for work and getting it warm (and believe me, those 2x5 are quite an effort for me )
Then i do 3 rounds of 3 minutes of bagwork. And with bagwork i mean punching, punching, punching punching punching on and on and on and on!!!!
I started out with just keeping my rithme, and if possible put some strength in it. And that wasn't easy. halfway the second round i was already beat. At first it was about being able to keep going. And as my trainer pointed out: "Points in de bout are made by hitting, not by how hard you hit. Hitting hard makes it worse for your opponant, but being able to hit hard, very hard, is a bonus which you have to earn by working hard for it!" After bagwork Then the normal training starts, which consists of one on one punching and/or sparring. After the training hour, i go one more round on the bag, before the cooling down.

After just 3 weeks i noticed that not only the punching-pace was much higher and i could keep going all three rounds, I was also able to punch harder and harder.

This greatly helped me in the ring. I can keep up all three rounds, and i get my breath back real quick. The minute of rest is more than enough, after that minute i feel practically fit again. And it really paid of in my next bout, my opponant sometimes flew through the ring after a simple punch, and started to get scared to get near me
Next week i'm adding one more round of bagwork to it. just to get it better.