"It's not an addiction. I just like running and I like to challenge myself. It's not just about the running too. It's about travelling all over the world, the people, the places ... the adventures," he explains.
A friend and fellow marathoner, Kiri Price, says Bill is one of 39 members of the NZ 100 Marathon Club. He's also among half a dozen members who have done more than 200 marathons each.
But more extraordinary, she says, is his capacity to encourage other runners. His is a welcome face at marathons.
At home he has so many medals, race T-shirts and certificates he "can hardly get into" his third bedroom, nicknamed "the running room".
Bill jokes he might have to get the T-shirt logos made into a giant eiderdown. He can't give them away because you have to earn them to wear them, he reckons.
As for the medals, these will one day weigh down his coffin.
Bill was born in Owaka, South Otago, and raised in the tiny Central Otago settlement of Omakau.
He used to run 4km to school and was always horse-riding, duck shooting, swimming in the river, building rafts or fishing - "a bloody good life".
Bill has two children, Matt and Kate, and five grandchildren, but these days lives solo. He retired as a social studies and geography teacher at Southland Boys' High School two years ago.
He did his first marathon, from Riverton to Invercargill, in 1980 in 3 hours and 11 minutes. He went on to shave this marathon time down to 2 hours, 50 minutes in Christchurch, then aged 46.
He ran "just" 29 marathons in the first 20 years. He then did 10 in 1998 on a year off touring the world. Since then he has done up to 25 annually. This year's tally is already nine.
He has repeated many - Boston nine times, the Southland Marathon (Riverton to Invercargill) more than 30 times and Buller Gorge 15 times.
Today, he'll clock up his 13th Rotorua Marathon.
Favourites include Boston ("the premiere marathon in the world" - you have to qualify to enter), the Riverton to Invercargill run ("a top course") and the Buller Gorge race ("amazing scenery and people).
Then there's the Midnight Sun Marathon above the Arctic Circle in Norway, the Niagara Falls run in Canada and an American run during which competitors cross a border.
Fast Kenyans inspire Bill, but so do those at the back of the pack who struggle for hours longer than most and often in changing weather conditions.
"Without them there is no race," he says. "Marathons are just not about the elite, the winners. Without the rest of us there is no marathon."
Bill can still run a marathon in 3 hours 40 minutes, regarded by most as a great time.
But he doesn't care about the time he takes nowadays, preferring to help others along the course complete the challenge.
He carries only a banana and a few barley sugar sweets for each race, nothing high tech.
He feels thankful to have had only a few injuries - a twisted knee in the Gold Coast Marathon in 2009, a pinched nerve at the Southland Marathon in 2011, plus "wear and tear after 35 years-plus running!"
And of course he wants to keep going "while I'm above ground".
"It feels like heaven on earth, especially at my age," he says.
So how will he celebrate his 250th marathon milestone today? Planning his next one, of course.
Bill's marathon tips:
• Train properly, have "miles in the legs" (he trains 14 to 16 hours a week).
• Don't bolt at the start or "you'll surely be in hell".
• Be patient at the start; use thoracic breathing, relax, focus.
• Drink water at each station.
• Eat a diet low in processed sugar.
• Don't think a marathon is beyond you. Remember "it's a mind game''.
