Joey Maloney (21), a physical education student from San Francisco, Felipe Saltz (21), a law student from Los Angeles, and Gemma Tomsett (21), a history and economic student from Tapanui, originally entered to run the half-marathon.
But the trio took inspiration from movie character Forrest Gump following a 30km training run just six days out from yesterday's event.
They decided to upgrade to the full marathon.
With shouts of "Run, Forrest, run" echoing in their ears, the trio completed the 42.2km course between Harington Point and Port Chalmers in 5hr 26min 20sec.
"We're [Maloney and Saltz] only here for the semester, so we thought it was all or nothing," Maloney said.
"I never thought that when I arrived back in July I'd ever run a marathon.
"It was kind of spontaneous really.
"But if Forrest can do it, we can do it."
Maloney has a background in rugby. He played fullback for Occidental College, in Los Angeles, and turned out for Harbour's premier two side in Dunedin.
Saltz has a background in basketball and football in Los Angeles.
"This is it. I'm going to retire after this," Saltz said. "This is my peak. My Everest."
Tomsett will miss both Maloney and Saltz when they return to the US in November. "I'm probably going to let myself go after this," she said.
She said one of the goals in getting to the start was stopping Saltz from partying.
The secret to getting to the finish was carbo-loading on pizza and pasta, along with watching Forrest Gump for motivation.
