The first thing that needs to be understood about conditioning is that literally every single person out there has the ability to fight from beginning to the end of the fight without gassing out, the only difference is the pace that they’d be able to maintain if they want to do so. In other words the work capacity is equal to the amount of energy stored and how much time it can be expended till you are “gassed” out.
This is because when it comes right down to it, just about everybody can walk a mile, but only a very small handful can run it in less than four minutes. This is why we train in intervals and constantly alter the workload within those intervals. The body can become efficient in a short period of time, but to effectively burn calories we want the body to remain in a state of “confusion”. By training our body to deal with a state of confusion we then can become conditioned to perform in many different situations “workloads”
Along the same lines, every fighter could make it through three or five rounds, but only those with a high level of conditioning can make it through while maintaining a high level of power throughout.
To find out more about conditioning check out this article from 8weeksout.com https://www.8weeksout.com/2015/06/09/3-tips-to-stop-gassing-out/