The elder statesman of the Labour Party lashed back, calling the comfortably-padded Slater "blubber boy'".
"I bet he is too chicken," Mallard said.
Well, he did accept.
And Slater - known for obsessively hounding issues - has turned his compulsive nature to the race and cycled about 15kg off his frame.
Mallard noted his opponent's weight loss on television.
"I'm distinctly lacking in confidence," he said.
"I had a major injury. I can cycle slightly better than I can walk."
Mallard, Labour's campaign manager for November's election, broke his right femur and right shoulder blade when he brushed wheels with another cyclist during a race in Otago five months ago.
Since then, he said he had considered withdrawing from the challenge and refusing to compete. But he has cycled the 20km course - of which they will ride three circuits - and says he will be there tomorrow.
"I'm probably of the mindset and generation that doesn't like quitting."
Slater, who Mallard calls an "obsessive character", is relentless.
"He is a cripple. And he's running a crippled campaign.'"
Slater has been in training and, as his physical fitness improved, so did his mental health.
Slater had publicly struggled with depression, and credits getting off anti-depressants, good vitamin B levels and a good diet with the improvement.
He conceded his mental state impacted on his blog with some "quite vile" writing.
"There's a lot of things I shouldn't have written or regret writing. But you have to own your words. I leave them there on the blog as a lesson to myself and other people as to what can happen if you get into the pits of despair."
Otago University zoologist Philip Seddon said whales in the wild would always be faster than ducks.
"Almost whatever kind of whale you thought about," he says.
Seddon - who runs the university's Wildlife Management Programme - said smaller whales were faster.
Slater's time could, perhaps, dictate whether the blogger was truly small and dangerous.
"Maybe he's an orca... a killer whale," said Seddon.
* The race starts at 1.30pm tomorrow, at Musick Point reserve at Auckland's Buckland Beach.
