ERIE, Pa. -- A teen girl believed to be the youngest to swim across Lake Ontario two years ago is now taking on an even bigger challenge: a marathon swim across Lake Erie.

Annaleise Carr is expected to start the 75-kilometre trek -- one and a half times her last feat -- early this morning at Presque Ile State Park in Erie, Penn.

The record-breaking swim, a fundraiser to send kids with cancer to camp, will end in Port Dover, Ont., and is estimated to take about 30 hours.

Members of Carr's support team say they're hoping for good weather since poor conditions could seriously hinder her efforts.

The start time has already been moved up in an attempt to avoid possible thunderstorms Saturday afternoon.

Her team says the 16-year-old has been swimming six to eight hours a day to prepare.

They say she is feeling confident and can't wait to get started.

"She's such a calm girl. Before her last swim, she was just as calm before she went into Lake Ontario," said Mark Ghesquiere, a physician who will follow her during the swim.

"Now she's two years older, more mature, she's better trained than she was then because she's got that much more strength to her, so I think even though it's a longer swim, if anything, she's going to do very well."

Carr's 2012 swim raised $115,000 for Camp Trillium.