RE: HIIT and "steady pace" endurance running.
It is not a question of either/or.
Good regimes to BOTH.
Both have their merits and both make a fighter have better cardio-vascular endurance and maximize VO2.
BOTH.
There is no reason to do one over the other.
Boxers, for example, have done both for ages:
Steady road work and HIIT bag work.
MMA fighters do both, pretty much every sport combat systems does a form of both steady ( paced) cardio and HIIT.
Psalms 144:1
Praise be my Lord my Rock,
He trains my hands for war, my fingers for battle !