By Patricia Sellers

Building a startup can be a 24/7 assignment. Building a high-flying startup can be, well, more consuming than that.

Which is why Susan Lyne, the CEO of fast-growing online retailer Gilt Groupe, is switching jobs with Chairman Kevin Ryan.

Gilt announced the new set-up yesterday, and by the time Lyne called me at 6 p.m. to explain, she had finished a full day in her new role. “We were talking most of the summer,” she told me, explaining that as CEO, she felt too buried in the weeds to see how and where Gilt should expand.

“The question was, how do I extricate myself enough to be able to work on big initiatives?” she said. “Over dinner six weeks ago, we came up with the idea of Kevin and me switching jobs.”

Simple enough, since Ryan has been a CEO of a fast-growing startup before. He ran DoubleClick for nine years and sold it to private equity for over $1 billion — before Google eventually bought DoubleClick for $3.1 billion.

While Ryan now concentrates on the day-to-day at Gilt, Lyne is charged with scoping the opportunities. And what are they? For one thing, she says, “We can do more with our private sale platform.” She mentions a recent coffee sale to Starbucks’ most loyal customers — those who have bought at least 35 “rewards” cards. Those 650,000 or so Starbucks loyalists got access to Gilt’s private sale, scooped up Gilt’s entire coffee inventory within a day — and a few thousand of them, Lyne says, signed on as new Gilt members.

Gilt started three years ago in the luxury apparel space, but she envisions doing more sales with mass consumer companies like Starbucks. She says she also wants to expand in full-price retailing, where Gilt has experimented with men’s and women’s apparel.

Lyne began her career as a journalist, as managing editor of The Village Voice and then founding editor of Premiere magazine, before she co-headed ABC Entertainment at Disney and then became CEO of Martha Stewart Living Omnimedia . So no surprise she’s interested in producing content as well as pure sales. Gilt’s first foray into content: the new Gilt MANual, aimed at men of style — or men who wish they had it.

Gilt is a remarkable growth story: $170 million in 2009 revenue is expected to increase to $400-500 million this year. The three-year-old company has raised $85 million. Is an IPO coming anytime soon? “Not this year,” Lyne says. “I would say that the earliest would be a year from now.”