Honestly though having a six pac doesn't mean you have great cardio does it? his brother never did and showed incredible cardio. I suspect the difference is a combination of Alek not training quite as hard plus being bigger and more of a brawler, you throw the kind of power shots he did vs Barnett for 10 mins for example and you will slow down. He does seem like he's held up physically better than many guys from his era, probably because he's not been fighting/training at as high a level. That said his grappling really looks like its regressed and I suspect would be exposed by anyone that good.