But when dusk settled Tuesday and the Portuguese Man 'o Wars began to swarm over her body, their tentacles clinging like invisible string and issuing painful stings to her face, back and arms, she could stroke no further.
Palfrey was pulled from the water at 6:31 p.m., nearly seven hours and about 20 miles into the swim. Red welts streaked her skin. Facial expressions of disappointment and pain co-mingled.
But hers remains an inspirational story.
Palfrey, an Australian whose accomplishments include a two-way swim across the Strait of Gibraltar and a marathon crossing of California's Santa Barbara Channel, is the embodiment of perseverance.
She tried the same Hawaii swim last April and the same thing happened, 12 hours into the attempt. And she'll try again someday, perhaps with better timing, for the simple reason that the Kaieiewaho Channel is the last of Hawaii's major channels to be uncrossed by a swimmer. Only one other person, Jonathan Ezer in 1976, has even attempted the crossing.
Palfrey, who twice has encountered great white sharks during ocean swims, had hoped a November attempt across the Hawaiian channel might be more suitable in terms of dangerous creatures.
It was worse. Last April she was not stung until several hours into the journey. On Tuesday she was stung for the first time, on her right arm, 90 minutes after stroking beyond the shore of Waikiki.
She was stung at least once every two hours until the sun began to set and the poisonous jellyfish became more active.
During the day, those with her aboard a companion boat, used kayaks and paddleboards to get closer and shout words of encouragement.
But dusk came and the situation worsened. Palfrey said the attacks became relentless and her swim unbearable when jellyfish were draped over both arms, strung between her fingers and slung like goo in her face.
"I couldn't even see because they sticking to my face and around my head," she said, adding that the most painful sting was to one of her ear lobes.
The swim was to have lasted through the night and long into Wednesday. In retrospect, Pelfrey said perhaps the jellyfish episode saved her from a nighttime assault by cookie-cutter sharks known to frequent the deeper parts of the channel.
Palfrey, though still in pain, on Wednesday afternoon was scheduled to appear for a press conference at the Waikiki Parc Hotel.
Asked by phone about her next challenge, she paused briefly, as if it were too early even to think about, and said she's scheduled to attempt a 26-mile crossing of the Molokai Channel next March.
By then most of her welts will have healed.
-- Images showing Penny Palfrey after her jellyfish ordeal (top), during her swim before it became too unbearable (middle), and welts on her arm are courtesy of Open Water Source