Lets say Iron Man Triatheletes are the pinnacle of fitness. How sustainable is that exactly?
In my mind, not very. From a physcial standpoint, there would be injuries that you would have to recover from doing a Triathelon every week. Not to mention the time element. The human machine being what it is, I am quite sure someone could adapt their body to do such a hellish feat weekly. There would have to be some downside.
It is said that Michael Phelps trains for six hours a day, six days a week, without fail. He swims approximately 50 miles each week, which is over 8 miles per training day. He has two massages everyday and also takes ice baths to help his body to recover.
Note two things, he does take time off from training, second, his career as an Olympic level swimmer is going to end earlier then later. In other words, he will not be at "peak fitness" most of his life. Obviously.
The extreme opposite would have to be laying in a bed, possibly eating at Phelps level of intake. So there is some grayish area of being fit that is reasonable. And some modicum of activity to maintain this.
In my mind, being in at the fitness level where I could run a marathon given a month of training.