If you would examine the training regimens of the great players of the 1950's and 1960's, you would find that most of them were in superior shape to today's players. They learned to do rigourous weight-lifting schedules, five-mile runs, track sprints, much of which was introduced by Harry Hopman, Hoad's back broke down by doing pushups with fifty-pound weights on his back.
There was one player who could grab the front legs of a chair while someone was sitting in it, and lift the person up to eye level. (Hoad did this once in a bar to an obnoxious heckler.)
These players did not have tie-breakers, and would play extended five-setters until someone became weary.
On the old pro circuit, with only six or eight of the world's best players, you had to play a tough match every time out. Imagine if the top eight players today spent all year playing no one but each other, and played about two hundred matches each. This would give you some idea of the old pro circuit.
Notice how many times they had to towel sweat off their faces back then, look at the films if needed.
Everyone knows what Lendl brought to the game in the 80's, fitness to a new level.
What this means is that he did physical workouts beyond what was done before him in most cases. But today the players are doing a lot more than Lendl and therefore more than what was done before him.
Todays game takes a lot more out of players than any games back in the 50's or 60's, looking at the films will tell all.
Not only are the practice workouts hard now, the game itself is much more intense physically.
Besides all of the above, I like to hear about the players of the past.
I know they made tennis today possible with their dedication and doing the best they knew at the time.