Marilyn Korzekwa's "biggest concern" as she prepares for a 28-km swim isn't the temperature of the water or the prospect of developing a killer cramp.

Rather, the 56-year-old Hamiltonian is dreading how much sleep she's going to get in before she takes the plunge into Lake Muskoka on Friday.

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Marilyn Korzekwa is attempting to swim across Lake Muskoka on Friday. (Supplied)

"I'm going to get like two hours of sleep," Kozekwa told CBC Hamilton on Thursday afternoon. "We have to get up at like three o'clock for the 5 a.m. start, so I'll be starting out tired."

But the psychiatrist is confident she'll have enough motivation to pull through. She's using the marathon dip, which she expects to last 12 to 14 hours, to raise money for the Good Shepherd, a social service organization that provides support for Hamiltonians who face poverty, homelessness, mental illness or other challenges.

"A lot of my patients need a lot more support than just seeing a psychiatrist," said Korzekwa, who practises out of St. Joseph's Healthcare Hamilton. "The Good Shepherd is really great at giving them the support that they need in terms of housing, health, emergency and crisis services."

Alan Whittle, director of community relations and planning with Good Shepherd Centres, said it's "really wonderful" that someone like Korzekwa "puts their body, heart and soul into supporting us and other organizations."

English Channel

This isn't the first time Korzekwa has embarked upon a lengthy swim to fundraise for the Good Shepherd. In 2011, she swam the English Channel, a 35-km, 16.5-hour ordeal, raising $8,000 for the charity. 

Video of Korzekwa's 2011 swim across the English Channel:

"It's fantastic that we have supporters like this who keep giving and giving year after year," Whittle said. "That's how we can keep making a difference in this community."

Korzekwa isn't attempting the feat solely for humanitarian reasons. It's also part of her training for a longer swim later in the summer. She's planning to cross the Catalina Channel in California, paddling 32 km between Santa Catalina Island and a beach just south of Los Angeles.

If Korzekwa is successful, she will become the oldest Canadian to have completed the deed.

"I thought I'd do one more big swim before my body gave out," she laughed.

Lifelong swimmer

A lifelong swimmer, Korzekwa was a competitive athlete through high school and university. She swam Lake Ontario "the easy way, which is south to north" in 1983 and then the opposite direction in 1984. She was the first person to make both trips.

After her Great Lake success, Korzekwa "kind of retired" from long-distance swimming, but continued to participate in the occasional jaunt for charitable causes.

She said she tries to focus her thoughts on her patients when she's in the water, as well as on her family and her supporters who will be on a a boat nearby to cheer her on.

"It's a little of everything, but I try not to think about the actual swimming."

Who knows? Maybe the lack of sleep will help.

To follow Korzekwa's progress or to contribute to her campaign, go to her blog, Swimcatalina2013.blogspot.ca.