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On October 12, she will realise that dream - swimming, biking and running more than 226km with nearly 1800 other athletes.
It all began when she was 12 and living in Invercargill.
''I caught glimpses of a local guy training [for the same race] and thought it was an incredible distance, so it was always in the back of my mind to do.''
Williamson, always an active person, decided three years ago the time was right for her to begin a rigorous training regime that would get her in shape for the ironman.
She trains for about 20 hours a week but says it is not often she finds herself struggling to get out of bed in the morning.
''It's a goal that I've been working towards for quite some time ... it is just a matter of fitting it in around the kids.''
Sometimes, when she is a bit tired or reluctant to go for that 4hr bike ride or 2hr run, it is her boys, Rocco (12), Vinni (10) and Luca (7), that help pick her up, offering encouraging words, accompanying her on swims or biking with her as she runs.
Seeing what she is going through is a valuable life lesson for the boys, she reckons.
Husband Tony, too, has been very supportive of her journey, but she said the hardest part of her schedule was finding time to spend with him.
''What I'm looking forward to most after this is to sit down and watch TV with Tony and not having to go to bed at 8.30pm.''
It is not just her family that has helped her achieve her goal. Williamson is grateful to coach Keegan Williams, mentor Bill Godsall, masseuse Theresa Kingsford and all the people that helped raise the $12,000 needed to get her to Hawaii.
Next Friday, though, she will be boarding the plane alone, which will not necessarily be a bad thing. Without distractions, she will be able to focus on psyching herself up for race day. That will include visualising various parts of the race, including the mass start, which is ''one of the greatest stresses of the race for me''.
With five ironman races behind her, including the Taupo event which qualified her for Hawaii, she knows how much the distance can hurt.
''The mind game is just as exciting as the physical challenge, to keep pushing when you are in pain.
''Ironman is a complete personal challenge. To do the best you can, it's a mind game. You've got to stay positive all the time and you've just got to think in the moment.''
Though her family members will not be in Hawaii with her, they will be glued to the television and computer at their Alexandra home, watching the coverage and following the live tracking to see exactly where she is.
So, after she conquers the Hawaii ironman, what is next?Family time, family vacations and, one day, the Coast to Coast.
There are 51 other New Zealanders competing in the race, including Gina Crawford, of Wanaka.
Julie Williamson: Training schedule
Monday: Morning, 1.5hr on wind-trainer; afternoon, 1hr-1.5hr swim.
Tuesday: Morning, 1hr-1.5hr swim; afternoon, 1.5hr-2hr hilly run.
Wednesday: Morning, 1hr-1.5hr swim, 4hr bike, 1hr run.
Thursday: Morning, bike ride, hill repeats; afternoon, run.
Saturday: Long bike ride followed by a shorter run.
Sunday: Easy swim.
Race day: 3.9km ocean swim followed by a 180km ride and a 42.2km run around the barren lava fields of Kona, Hawaii's big island.