People.com said Kate looked slim and relaxed as she helped kick off the 135-mile, three-day "Ring of Fire" race in the coastal town of Anglesey, Wales.
Her husband was by her side for the event but their infant son stayed home with his maternal grandmother, Carole Middleton, the report said.
Kate was quoted as calling her son "sweet" and saying he is sleeping well.
The royal couple lived in Anglesey before the birth of their son. They have since been splitting their time between Kensington Palace in London and the Middleton estate in Bucklebury, England.