Just got back from vacation there yesterday. Drove most of the island. Was thinking that although amazingly beautiful, would not be the best for riding. The roads on the south and west portions of the island are probably the best - shoulders or bike lanes for a lot of it. I hardly saw any people on bikes, and only saw two actual cyclists during the week I was there - and they were riding up to waimea canyon on mountain bikes. So you'll be the anomoly and I doubt drivers are expecting cyclists - so be sure to wear bright clothes and have a head and taillight. The roads themselves seem great - it's just the traffic and lack of side roads that will make it tough/dangerous.
That said - you can get in some fantastic long hikes and runs, and swim in lydgate which is probably protected enough that you'd be ok even in winter. I think there's a lot of dirt roads, so depending on the weather, a mt bike would be a good alternative.
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