While the 3000 meter race might seem a little daunting, Derek is no stranger to the water. A southern California native, Derek's a big fan of swimming -- and even credits it for her age-defying figure.
"I have to admit part of my compulsion to do this swim was I want to eat like Michael Phelps," she told ET in 2013. "I mean if you look at all the list of what we can do for exercise and how many calories it burns, swimming's always at the top. So that was one of the reasons I took up swimming again."
"Swimming is good, swimming is really good," she added. "I do recommend it."