On Friday, Kate, 31, made a surprise appearance at the Ring of Fire race in Anglesey, Wales.
Looking slim and relaxed, Kate chatted with locals about babies, pregnancy and the race, telling onlookers that her five-week-old son, Prince George, was home napping with his grandmother, Carole Middleton.
The new mom described her son as "sweet" and added he was sleeping well.
Dressed in a patterned silk blouse by Zara – which she wore while visiting the Swiss Alps in March –, a green Ralph Lauren blazer and slim black pants, Kate glowed as she and husband Prince William kicked off the 135-mile race along the coast of Anglesey, where the couple currently lives.
Supporters were bowled away by the appearance of the couple, especially the popular Duchess of Cambridge.
"The Duchess is absolutely beautiful, lovely," said Andrea Hall, whose husband was running the race. "She looks amazing, I can't believe she had a baby only a month ago."
Lisa Hirst, of St. Helens, said both Kate and William looked relaxed. "She looked beautiful, isn't she beautiful?" Hirst gushed.
Sophie Bingham, a mother herself, said Kate didn't look as if she had a baby. Kate, she said, looked relaxed and acted genuinely interested in everyone. "I'm surprised, as she's probably been up [in the night] a few times," Bingham said.
Standing on a grassy hill overlooking a rocky bay and backed by misty crags, Kate and William first met with runners and race organizers, before starting the race, asking about the difficulty of the course.
Race directors James Bingham and Quentin Brooksbank said it was an absolute honor to have the young royals kick off the race, which is only in its second year, and said they did it because they wanted to give something back to Anglesey and Holyhead.
"They supported what we're standing for," Bingham said.
Kate joined the crowd in counting down from 10, before William rung a large bell and the runners took off in a pack.
"The circus is off!" William exclaimed as the runners passed him and his wife.
Theresa Schaludek, from Vienna, had tears in her eyes after Kate and William walked past her. "She's beautiful," Schaludek said quietly. "You don't have a chance to see them, especially from Austria."
Sharon Bowen, who lives in North Wales, shouted "Thank you for coming!" to William and Kate, who acknowledged with a wave. "I think it's great and it's so lovely to see them in such a lovely setting," Bowen said. Adding that she was stunned at Kate's natural beauty, Bowen said of Prince William, "he's a lucky man."
The race attracted 92 runners from as far as Canada and the Canary Islands, and included five local runners, race organizers said. Over three days, the ultra marathoners will encircle all of Anglesey Island.