IN the spring of 1976 I took myself to the first available screening of ''All the President's Men,'' and sat enthralled in the darkness as Robert Redford and Dustin Hoffman and Jason Robards portrayed the men I only wished I could be. When the lights came up at the conclusion, I discovered several of my journalistic colleagues dotted around the cinema, slumped thoughtfully in their seats. Our eyes met glancingly: we all knew what we were covertly thinking. If only. . . . And one day. . .