"[We've been together] since February," Walsh, 39, told PEOPLE at a poolside party for facialist Kate Somerville at the Beverly Wilshire hotel.
"I know – I'm literally living the dream. But you know when you know. I was not expecting it, so it's kind of amazing."
While Walsh says she loves "everything" about her fiancé, a production co-president at 20th Century Fox, she admits she's partial to his looks. "He's pretty handsome," she said, adding with a giggle, "He's ridiculously handsome!"
And it doesn't hurt that he's helping with the wedding plans – well, in his own way: "He's really great. He's there, but he's not in the way! He wants to contribute."
So, she's taken charge. "You start getting into this and you don't want to be a cliché, but then suddenly you're like, 'I'm doing all this stuff!' "
To help her manage things, she and Young are taking a quick vacation. "We're going to a balmy island," she said. "This is my getaway, and as soon as I come back we just hit the ground running."
Reporting by JENNY SUNDEL