Good luck, Ivan.
Anyone taking this on (and the audax version) has my utmost admiration. The constant ups and downs, as well as the twists and turns, riding 100 miles on the ground to cover 25 as the crow flies, plus plenty of sideways rain, must make this a very tough event.
I'm sure you know this already, but riding a 600 fresh is a lot easier / quicker than riding one after 2 or 3 others. If I were riding it, I would expect to do less than 300km per day after day 2, so I'd be looking for 400+ on day 1.