I suppose if you spend all of your day in front of a computer, you'll have difficulty understanding what the human body is capable of on its own with enough time and discipline and you'll just assume everyone is taking shortcuts. Najani isn't some special case; he's a prototypical example of "easy gains" that pretty much any guy can get with a year of dedicated training and diet if you're focused on muscular appearance rather than muscular performance. (For those who understand the lingo, this means he focused heavily on large fast-twitch muscles which grow quickly and easily, at the expense of the slow-twitch and medium twitch muscles that would relevant for most athletic performance.)

Members of the military routinely do 12+ hours a day of physical exercise, including hours of cardio, and yet none of them have problems disappearing (and only about 2.5% of military personnel use steroids). Triathletes, similarly, can spend 6 hours a day or more on exercise.

Cardio doesn't consume muscle unless you combine it with starvation. Muscles are the last-ditch choice of fuel. On a proper diet, cardio burns glycogen and fat. And if you're as fast as Pratt was when he started his that exercise regime, you've got plenty of fuel for hours of daily exercise, no steroids or hormones required (except for the ones your body generates itself in response to exercise and diet).