Breaking the two-hour marathon barrier would be "like stepping on the moon", says Eliud Kipchoge, who has "no doubt" he will achieve it in his next attempt.
The Kenyan, 34, hopes to be the first athlete to break the mark in the Ineos 1:59 Challenge in Vienna in October.
He holds the world record of two hours one minute 39 seconds but this attempt would not be an official record as he will be assisted by in-out pacemakers.
"I have no doubts at all," said the Olympic champion.
Kipchoge, who won the London Marathon for the fourth time this year, clocked 2:00:25 in a similar unofficial event at the Monza Grand Prix circuit in Italy in 2017.
This latest attempt will take place in the Prater park in the Austrian capital, with Kipchoge running laps of a flat 9.6km course that includes two 4.3km stretches.
"(Breaking the two-hour barrier) is like stepping on the moon, going up the tallest mountain and even going to the middle of the ocean," said Kipchoge.
Kipchoge set his official world record in Berlin in September 2018, with athletics' world governing body, the IAAF, not recognising his run at Monza due to the use of pacemakers.
Small groups of pacemakers ran pre-defined segments of the circuit before handing over to another group, and the Kenyan did not have to slow down for feed stations as drinks were delivered by scooter.
Kipchoge says he does not mind that his time will not be officially recognised if he does run under two hours.
"It's about history, about leaving a legacy, I am going to make history and leave a legacy, it's not about making a world record," he said.
"I am already a record holder for marathon - this is for the human family."