One particularly harrowing practice set stands out to Katie Ledecky and Simone Manuel when they think about training in a two-lane backyard pool for three months this year. Their coach, Greg Meehan, called it "Death by Relay."

The two American Olympians — who combine for 10 Olympic medals, seven gold — alternated who was swimming at an as-fast-as-possible speed, and when one touched the wall from her 25-yard lane, the other dove in.

"It was just a burner of a swim set that was actually quite fun and something very different for us," 23-year-old Ledecky said. "But it was super challenging for...