G'day Sarah,
Welcome to the forum.
I did the Triple Top a couple of years ago after a friend talked me into it. (What is it about friends?!)
Part of taking up the challenge for me was to have a very concrete goal to get as fit as possible for big summer walks. The said friend kept saying, "It's not a bushwalk. Don't think of it as a bushwalk. It's not a bushwalk!", and she was right. She also mentioned that I wouldn't be stopping. "Not stopping?????!!!!!!" I questioned in astonishment. Reality was, I didn't really stop - except once, just to get my hat when the sun came out. And of course, there were tiny stops on the way up Van Dyke to let runners go past. I even overcame my sense of wrong to 'just throw the cup on the ground', as the very helpful drink station folks encouraged us to keep going. And the orange skin. (Yes, they told us they'd pick them up too.)
There's a great atmosphere on the day, but it is totally different from bushwalking. It surprised me that I ended up passing much fitter people. My conclusion was that being used to rough tracks/mud/rock scrambling helped me make up time.
I was less minimalist, and was happy with my decision. I had a light backpack cinched right in against my back, with about a litre of water, basic first aid, a light rain jacket & pants, ear muff and gloves. I had snacks in my top pockets, which I munched on while I walked. (My friend took a bandage and 300mls of water, plus pocket snacks, and was happy with that.) Before the day, I tried a couple of different options, like wearing a waist bag at the back. But it bumped around too much when I trotted, so the pack worked better.
If I hadn't snuck in under 5 hours, when my goal was under 6, I'd probably be doing it again this year.
It was a great experience, despite being middle aged and not being a runner. Many walkers, like me, take a walk-trot-walk-trot approach. Walk uphill, but not so fast as needing to stop, then trot the flats and the downs when possible.
You've got a bit of time, so start some mountain/hill walks if you haven't already. Get as fit as you can by doing exactly what you'll need to do on the day. If you live up this way, and can go up Claude sometime soon, that'd probably give you a good idea. Only 2 more mountains like that one! So don't start out too fast. Have fun, in a bizarre kind of way.