You're right there, and it has to be a trained quality. You'll see alotta fighters just take a 30 minute jog or a slow plodding pace on the jumprope, maybe half intensity on the bags and maybe just going through the motions when they shadowbox. Those kind of people don't go far in the sport.

The key to building stamina/endurance to work at your capacity and to build on it. It's important to establish a baseline so you can improve on it.

Even a drawing board like shadowboxing it's not like looking at the clock and saying to yourself "ok, 3 rounds of air punching" You gotta take it seriously and practice different unnatural things until they become apart of you. This applies for learning techniques, increasing your intensity with consistancy of progress, and chaning your attributes and qualities.