How to train for a DOUBLE century (200 mi or 320-350km) on one day?

This is the link to Audax riders thread.
You are correct going from 250Km ride to a 350Km ride may sound only another 62 miles. From my Audax experience, feeding/ hydration and electrolytes are critical. Not burning your matches too early in the ride. You do not say how much climbing will be in the ride. Typical rides i have done have 2500-3000m of climbs. I have done PBP with 11000m of climbs and 1240km.
My FTP is 250w and 75kg. The 27km/hr average sounds high if its a hilly course, if its flat and light winds should be doable. Start as early as you can (Early Audax is 6am) I have done 400km (Hilly) unsupported in about 18hours and carrying all my gear, and stopping in cafes and restaurants. Could have done this quicker, but sitting in one of the controls 10pm and heavy rain much nicer another coffee and cake. Plan for some low points.

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