Hey Kiwi Grok, fantatstic goal. I biked round lake Taupo a few years ago - it is a LONG way !!!!!. I also have VFFs and have only done about 12km as the longest - so far. However that was totally on the road. Seaweed is right not so great when on the road, and yes my calves need a heap of streatching first.
I am really keen to do the molesworth 84 km, but that is all shingle - so will have to look at a more substantial shoe for that.
Anyway good luck in your quest
"never let the truth get in the way of a good story "