Kipchoge is preparing to compete in Sunday's Tokyo Marathon in his first race since Kiptum was died on February 11 at the age of 24.
The 24-year-old world record holder passed away months after beating Kipchoge's record and the two were expected to face off for the first time at this year's Paris Olympics.
The 39-year-old Kipchoge said it was "unfortunate that he left here,” referring to Kiptum.
"His career was in high spirits, and he was really running on a high level," Kipchoge said.
Although Kiptum only competed in three marathons, he won all of them and posted three of the seven fastest times in history. He was driving in the Rift Valley, the heartland of Kenyan distance running, when his car careered off the road.
Police said Kiptum and his Rwandan coach Gervais Hakizimana died on the spot while a woman passenger was injured. Their death shocked Kenya and the world of athletics.
Kipchoge said this year's Olympic marathon would be "a little bit different" following Kiptum's death.
"There were high expectations," said Kipchoge, who won Olympic gold at the 2016 Rio Games and again five years later at the pandemic-postponed Tokyo Olympics.
Kiptum ran a time of two hours and 35 seconds at the Chicago Marathon last October to slice 34 seconds off Kipchoge's previous world record.
He stood on the podium alongside Chicago women's champion Sifan Hassan of the Netherlands, who is also competing in Tokyo.
Hassan, who won in Chicago in the second-fastest women's time in history, said she was "heartbroken" by Kiptum's death.
"When he broke the world record in Chicago I shared it with him," she said.
"He was so young and he was showing the world what was possible. It's very hard," she added.
Kipchoge returns to Tokyo for the first time since breaking the course record in 2022 in a time of 2:02:40.
His strongest challenge in this year's race is expected to come from fellow Kenyans Vincent Ngetich, Timothy Kiplagat and Benson Kipruto.
Reigning Olympic 5,000 meters and 10,000 meters champion Hassan is competing in only her third marathon, having won both her previous races in London and Chicago.
She will compete against Ethiopia’s world marathon champion Amane Beriso Shankule and Rosemary Wanjiru of Kenya, last year's Tokyo champion.
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