Cavendish, who this year claimed a record 35th stage win on the Tour de France, is expected to retire from professional cycling at the end of the year.
"I will always ride my bike, but I want to run now," Britain's Cavendish, who won the Tour de France sprint on the Champs Elysees in 2012, said in an interview with Men's Health.
"It's pure - get your shoes on and off you go! I've already agreed to do the Paris Marathon with my brother. He started cycling before I did, and he was better than I was.
"(In the) past four years, he's started to get fit. He's riding and started running. For the first time since we were kids, he's lighter than me."
Cavendish said he has not "entirely committed" to what he will do next but is focused on completing his year with the Tour of Saitama and the Singapore Criterium.
"I'm still racing this year, I'm contracted until December. I've got races in the Far East in October," said Cavendish, who rides for the Astana Qazaqstan Team.
"What is guaranteed is that I'm never doing the Tour de France again. That's a dead cert."
Cavendish hinted at a management role in cycling.
"I will always ride a bike, but the past few years I've known what I want to do after," he said. "I've set the wheels in motion for that. I want to stay in management in the sport, I still love it.
"I brought a lot of people to this team (Astana Qazaqstan) over the past two years, and I know what it takes to be successful. I've been building up to the moment I'm not racing."