"I hate it and love it at the same time," she says of running for her cardio routine

Credit: Matteo Prandoni/BFAnyc/Sipa

Stephenie Meyer is showing off a svelte shape thanks to a switch-up in her cardio routine.

“I used to be more an elliptical-and-weights girl but I’ve started running in the last year and I hate it and love it at the same time,” the Twilight author told PEOPLE at the New York premiere of The Host, a new film based on her 2008 science-fiction romance novel.

“It’s one of those weird things, there’s nothing really like it,” she added when asked about her weight loss.

Meyer, 39, looked lovely on the red carpet at Manhattan’s Tribeca Grand Hotel in a figure-flattering striped Kate Spade dress. But she says staying healthy is difficult while on the road promoting The Host, which hits theaters Friday.

“This tour has been awful for my diet. Really, really bad!” the mom of three said. “I have to get home and get back to my gym.”

She also said she has the support of a close family member: “My sister is a big runner and she’s so excited [I’ve started running]. She wants me to run races with her now.”

Although happy to return to her Arizona home, which she shares with her husband Christian Meyer and three sons, the author is proud of The Host.

“It’s this whole different place and a different kind of thought,” she said. “It’s a lot more serious. It’s a lot more of a broader spectrum of emotion. It’s my favorite of my novels so I’m kind of prejudice towards it.”