For starters he had no clue what he was doing, but he just went a milestone at a time.
This is a nice, short article about how Forrest Gump started his running spree. If you haven’t seen the movie (and you should), Forrest just starts running one day and doesn’t stop for three and a half years. He ends up running coast to coast across the U.S. several times for no apparent reason. Then one day he realises he was trying to deal with the death of his mother and best friend and suddenly decides to stop.
What’s interesting is how he started and how he achieved so much.
He just started. And kept going. One milestone at a time.