There are a few route options; one longer and hilly, one shorter and a little less hilly (but bear in mind that the IOW is not a flat county!), and then one gravel ride option too.
Meet in Ryde Ferry Terminal on the IOW for a 10am start. There’s parking at the Wightlink ferry terminal in Portsmouth and you can take the Wightlink Passenger Ferry from Portsmouth Harbour at 9.15am (22mins) as a foot passenger with a bike for £26.20 day return.
There is a Premier Inn at Portsmouth Historic Dockyard (10mins walk from ferry) with availability if you want to stay overnight before/after, or you could make a weekend of it and stay on the Island.