What does this quote mean? showing up is 80 percent of life

In 1989 Woody Allen was asked about this saying by William Safire, the language columnist for the New York Times, and Allen replied with a letter in which he asserted: "I did say that 80 percent of success is showing up." Hence, Allen accepted credit for a common variant of the expression using the word "success" instead of "life". The details of this interesting cite are given further below.