Olympic champion Eliud Kipchoge believes the Ugandan could become the world’s fastest man over 26.2 miles.
Cheptegei won 5,000m gold at the Tokyo 2020 Games and holds the world record for both that distance and the 10,000m.
His Kenyan inspiration, Kipchoge, is now backing Cheptegei before the 27-year-old’s marathon debut in Valencia on Sunday.
“He is already a record holder in other fields and he has a huge chance to break a world record in the marathon,” Kipchoge told BBC Sport Africa.
“Joshua is talented and disciplined. He is learning well.
“I am happy Joshua is trying a new venture. I will be watching and giving my moral support as always.”
Cheptegei is regarded as one of the best distance runners in the world, with three 10,000m world titles and a world cross country title alongside his Olympic gold.
He previously held the world records for the 5km and 10km road races and trained with Kipchoge, who broke the marathon record twice, in 2015 in Kaptagat.
The Kenyan says Cheptegei’s “growth” has derived from the various distances he has run, but the 39-year-old has advised the Ugandan to run his own race on Sunday.
“Kipchoge inspired me a lot when I was starting up my career, his legacy inspires me,” Cheptegei said.
“His kind words have always been able to shape me over the days and years. Eliud is always keeping an eye on me, always guiding us in a good way.
“It’s a great honour to learn from the great. For this special regard, I’m looking forward to putting into use what we always share together.”