"My initial reaction was ... F*** ... I was actually pretty reluctant to take the fight," he said.
Green has been training with boxer and former rugby league player Monty Betham.
"[Betham] gave me a bit of confidence and faith that we'd be ready to the point that I could take [Guildford] on and win."
The charity fight night aims to raise $100,000 for Waikato Hospice, and Green said the charity aspect, as well as the fitness, was what drew him to the event.
"Whether I lose in the ring I still win in the end," he said.
Guildford said he never watched The Bachelor NZ, but was aware of the show through social media while he was playing rugby for French club Clermont Auvergne.
When he returned to New Zealand from France he was overweight and since training with Ryan Whittaker at Napier Boxing had lost between 6kg and 7kg.
"I've been asked a lot in the past and I've said no ... but coming back from France it gave me a focus... that would make me better as a person and as a rugby player.
"It'll make me fitter, stronger and mentally tougher."
Guildford said the fight was "daunting" and the training so far had been tough.
"It's been bloody hard so far and it's not going to get any easier.
"The footwork and stuff ... I'm not the best dancer in the world, but it does feel like you're on the dancefloor sometimes."
Guildford agreed rugby players were often considered to have a bit of "mongrel" and said he wouldn't be "folding over" in the ring.
The charity fight night will be held at Hamilton's Claudelands Arena on December 5 and netball great Irene van Dyk has signed up to the event. Her opponent has not been confirmed.
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