Malcolm Hicks set the pace for the field of 2375 over the first half in 1h 13m. Joining Hicks were Lett, Tony Payne, Josh Maisey and Steven O'Callaghan. Maisey dropped off the pace at 19km and at 30km Payne maintaining the steady pace moved into the lead. But his bid for victory was short lived and 2km later was clutching his side with stomach cramps. Lett grabbed the opportunity and took off, building a lead with 10km to run. He finished in 2h 27m 38s.
Nick Horspool, of Wellington, came through well over the second half for second in 2h 28m 46s with Maisey third in 2h 29m 8s.
Lett said he couldn't believe his luck when Payne faltered.
"He was a wee way ahead and he was looking quite good and I thought it is going to be tough. But it is a long way to go and anything can happen and you just have to dig in and do what you can do and I was lucky that he slowed down a bit for me."
Beijing Olympian Liza Hunter-Galvan, 45, won the women's marathon in 2h 47m 42s. Jess Ruthe led out early but was forced to withdraw at 23km when her right calf muscle seized.
In almost a track race that came right down to a sprint finish, Camille Buscomb edged Commonwealth Games double silver medallist Nikki Hamblin to win the women's half marathon in a career best time of 1h 15m 50s. Hamblin was six seconds back.
National 1500m champion Hamish Carson won the half marathon in 1h 8m 27s, 20 seconds ahead of Daniel Balchin.