Apparently he smoked a lot of weed. There's some story about a guy tracking him down at the time and him having a bin full of it, also Jules Siegel, an old college friend, said he'd told him that he was "so fucked up I can't remember writing some of this stuff".
How on Earth could a single person create something so magnificant, so complex and well conceived. GR is like that for me. How did he do it?