Best Shape of Their Lives stories, as Harlan says, are "a rite of passage of the baseball calender" and have been around "since the invention of February itself." While his particular entry highlights the offseason work done by Ryan Zimmerman, John Lannan and Craig Stammen, you're going to see dozens more between now and St. Patrick's Day, and for every player who gets written up in such glowing terms there are probably a dozen others who could also be said to be in the Best Shape of Their Lives and just don't get the press coverage.
The fact of the matter is, there are very few ballplayers any more who don't take fitness seriously, and who don't take a professional approach to staying in shape in the offseason. The John Kruk and David Wells types of the baseball world are a dying breed, if not quite extinct. CC Sabathia might be the biggest big name left, and even he isn't a true fatty. Roger Clemens' workouts used to be legendary (chemically aided or otherwise) but when you hear Zimmerman compared to an NFL linebacker in his ability to handle grueling workouts and come back for more, it starts to sink in that what was off-the-charts just 10 years ago in terms of dedication is probably just above average today.
The baseball heroes of yore will always be put up on a pedestal, and rightly so, but is terms of pure athletic ability they're simply not in the same league as the guys playing today. It's a fact that usually gets forgotten or dismissed when people start talking about the "taint" of the "steroid era".