In the '60s he wrote innovative pieces in which he was both participant and observer. That writing had a profound effect on Tom Wolfe and Hunter S. Thompson and paved the way for the New Journalism. But, to his mind, the real challenge was fiction. In fiction writing, you're doing two things. One is you're doing your best to write very well every page, and the other is that you're making up the story, so you have a lot of particularly serious decisions to make all the time.
Novel writing is all decisions.
In his sixth decade of writing, Mailer still wants another shot at it and is willing to put in the requisite hard work. The truth is you have to get in wonderful shape compared to the shape you've been in if you're going to start a novel. In the past, before my knees gave out, I used to jog more miles and work out harder in the gym. You decompose a little physically when you work on a novel. Then you stop for a while and build up again.