"I'm very biased, but she's beautiful. She's got blue eyes and exactly the same colour hair as her father - kind of a browny colour. She's starting to chub up a bit now, she's very healthy."
The Letts, who both work as physiotherapists and both run marathons, are finding parenthood delightful.
"It's wonderful, you wake up in the morning to lovely smiles and cooing. I wouldn't change it for the world, she's so gorgeous. I keep looking at her and thinking how lucky we are to produce such a beautiful baby," Mrs Lett said.
Mr Lett said Ruby was a good baby, who needed feeding only about every three hours, and generally slept about five hours at night.
"It's really good, it's quite busy - quite a learning curve. Each week is a bit different."
After his daughter's birth, Mr Lett took a rare break from running.
In the lead-up to the Auckland Marathon he'd been doing about 200km a week, and in the four years that Mrs Lett has known him, she said he'd taken only one day off.
But after Ruby came along, he stopped running for two weeks.
"I think it was something to do with the baby, rather than just recovery," Mrs Lett said.
"In fact, his coach texted me and said, 'Ah, how's Stephen's running going?' and I texted back, 'I think Ruby has ruined his career'."
Mr Lett is back running about 150km a week now, training for his next marathon on the Gold Coast in July.
Ruby's parents said they hoped she'd develop a good set of runner's legs.
"I think Ruby will have a bit of determination," Mr Lett said. "She's still a bit chubby now, though."