The sevens dream might be over, but the song and accompanying video encourages Kiwis to "go for gold" in various sports including rowing and shotput.
It features Marshall in front of notable landmarks in Rio de Janeiro such as Christ the Redeemer, Copacabana Palace and the Escadria Selaron steps.
He said the song and video took a full Olympic cycle of four years to produce.
"It's been four years in the making. About two years ago my wife and I, my children and a cameraman went to Rio and we shot the video and went back to the studio and worked on it, and put it all together finally about five or six months ago."
Marshall lives in Araraquara in Sao Paulo, but says he prefers being in Rio because "it is beautiful".
"My wife's a Brazilian, I met her in London 20 years ago. We moved to New Zealand, and had a couple of kids there.
Together with his wife, Marshall runs an English-language school in his adopted home.
He has returned to Whangarei during the Olympics after being unable to get tickets to any of the events, and has spent the past two weeks travelling to schools in Northland playing his Olympic anthem.
"They absolutely love it, they like everything. I sang the song about 30 times at one school."
He's been keeping a keen eye on the performances of the Kiwi Olympians, and despite two silver medals he's still waiting for his motivational tune to have an impact.
"I don't want to put too much of a downer on it but it hasn't been too crash-hot."
Yesterday was another disappointing day for the Kiwi athletes. Cyclist Linda Villumsen, the reigning world champion in the individual time trial, finished a disappointing sixth.
The New Zealand men's sevens team are also out of the medals after dropping their quarter-final match against Fiji - their third loss of their campaign.
But hopes were high for gold on the rowing course overnight, when men's pair Hamish Bond and Eric Murray were due to contest their final.
"Super Saturday" could bring further rowing medals, while swimmer Lauren Boyle is set to compete in her favoured 800m freestyle event.
The big focus tomorrow will be shotput queen Valerie Adams, who is looking to make history as the first New Zealand woman to claim three Olympic gold medals.