Ricki Lake showed off a newly trim figure at a charity event in Los Angeles on Thursday, and credits a strict exercise regimen for the transformation. "I think I had [that muscle] under all that fat all those years. I think it was just there," Lake told PEOPLE at the Choose Your Cause Launch Party and Charity Benefit at Fred Segal Fun in Santa Monica. Her secret? "I work out with a trainer three days a week," she said. Lake, who is single, dated a personal trainer after her 2004 split from husband Rob Sussman. But don't expect similar sparks to fly this time. "He is totally hot, but he's 25, and I have to draw the line at 30," said Lake, who is 39. "Thirty is like my starting point." Besides, she said, she isn't actively looking to jump back into a relationship. "I'm just starting dating, and I'm not desperate, and I'm not lonely," she said. "I'm really happy being alone." In most recent project, Lake serves as executive producer on a new documentary, The Business of Being Born, about the maternity care system in America. "It looks at all that from my own personal experiences," says Lake, who has two sons with Sussman. "It's fascinating."