He proved that prediction wrong again on Wednesday.
In a skin-tight yellow and violet jersey, the 105-year-old Frenchman set a world record in the 105-plus age category -- created especially for the tireless veteran -- by riding 22.547 kilometers (14.010 miles) in one hour.
"I'm now waiting for a rival," he said.
Marchand had ridden faster in the past on the boards of the Velodrome National, a state of the art venue used to host the elite of track cycling. But he had warned before his latest attempt that his current form was not as good.
"I did not see the sign warning me I had 10 minutes left," Marchand said after his effort. "Otherwise I would have gone faster, I would have posted a better time. I'm not tired. I thought my legs would hurt, but they don't. My arms hurt, you have to hurt somewhere."
Three years ago at the same venue, Marchand covered 26.927 kilometers (16.731 miles) in one hour to better his own world record in the over-100s category.
Still, impressed fans and chanted "Robert, Robert" during the last minutes of his ride. Marchand received a standing ovation once he completed the last of his 92 laps and was then mobbed by dozens of cameramen and TV crews.
"He could have been faster but he made a big mistake. He has stopped eating meat over the past month after being shocked by recent reports on how animals are subjected to cruel treatment," Marchand's physiologist, Veronique Billat, told The Associated Press.
By way of comparison, the current overall world record for one hour is 54.526 kilometers (33.880 miles) set by British rider Bradley Wiggins in 2015. But Wiggins, who smashed the previous record using the world's leading track cycling equipment, is now retired.
Marchand, who lives in a small flat in a Parisian suburb with a meager pension of about 900 euros ($940), keeps pedaling and stretching every day. As if time had no effect on him.
"He's got two essential qualities. A big heart that pumps a lot of blood, and he can reach high heart beat values that are exceptional for his age," said Billat, a university professor. "If he starts eating meat again and builds more muscle, he can better this mark."
Marchand, a former firefighter who was born in 1911 in the northern town of Amiens, has lived through two world wars. He led an eventful life that took him to Venezuela, where he worked as a truck driver near the end of the 1940s. He then moved to Canada and became a lumberjack for a while.
Back in France in the 1960s, Marchand made a living through various jobs that left him with no time to practice sports.
He finally took up his bike again when he was 68 years old and began a series of cycling feats.
The diminutive Marchand - he is 1.52 meters (5-foot) tall and weighs 52 kilograms (115 pounds) - rode from Bordeaux to Paris, and Paris to Roubaix several times. He also cycled to Moscow from Paris in 1992 and set the record for someone over the age of 100 riding 100 kilometers (62 miles).
"If the president of his teenage club who told him he was not made for cycling because he was too small could see him today, he would kick himself," Marchand's coach and good friend Gerard Mistler told the AP.
According to Mistler, the secret behind Marchand's longevity relates to his healthy lifestyle: eating a lot of fruits and vegetables, no smoking, just the occasional glass of wine and exercising on a daily basis.
"He never pushed his limits, goes to bed at 9 p.m. and wakes up at 6 a.m., there's no other secret," Mistler said. "If had been doping, he would not be there anymore."
To stay fit, Marchand rides every day on his home trainer and puts himself through outdoor training sessions on the road when the weather is good enough.
"One needs to keep his muscles working," said Marchand, a faithful reader of communist newspaper L'Humanite.
"Reading a lot keeps his mind alert," Mistler said. "He does not watch much TV, apart from the Tour de France stages."
At 105, Marchand is not making plans for the future. His coach would not be surprised to see him back on the boards, though.
"Setting goals for himself is part of his personality," Mistler said. "If he tells me he wants to improve his record, I'll be game. Robert is a great example for all of us."