No stairs will cause both muscle strength and flexebility imbalances that will cause injurys, effect posture and hinder performance in the long run, itll tightn the hip flexors and this is where the problem will start to snowball if left unnoticed
Im not sure if im right and someone please correct me if im wrong. but juan diaz used the mountain climbing machine for most of his cardio and stamina workouts, i think he rarely ran. Wladimir klitchko does swimming i believe? I think it comes down to preference, i love running it helps me mentally and overall helps me with my breathing which translates well into boxing because if you breath properly you last longer. I have tried biking and swimming but it just doesnt cut it out for me
All of those distances if they're done continuously. The longer the distance the lower the intensity, the less relevant it is to boxing.
If you want to get fit, for overground running, and stay injury Free while building Muscle, Tendon, Ligament Strength plus C/V endurance, DON'T over ground Run :yep.