KAILUA-KONA, Hawaii—Sean Swarner remembers watching the Ironman World Championship when he was 13. He would watch the grueling race on television from a hospital bed while undergoing chemotherapy.

Swarner, who is now 34 and lives in Boulder, Colo., will attempt to finish the 140.6-mile endurance test Saturday. It’s the most recent goal he has set for himself as a two-time cancer survivor.

“I just want to let people see that cancer is just a speed bump in life,” he said Wednesday.

At age 13, Swarner was diagnosed with stage four Hodgkin’s Lymphoma, a cancer o...

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