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I was dead happy I rode an MTB, with 2.4" tyres. I rode with a guy a lot who was on a CX bike (Trek Boone), and he was happy enough with it, but he suffered on the rocky bits, and the washboard (we all suffer by degrees on washboard).
I didn't need aero-bars, or fast road tyres. I was cruising happily at 15-20mph on the pavement and hardpacked gravel on the level using Conti XKings.
Really the most important thing is a good saddle. I tried 4 different saddles in the 18 months leading up to the race, settled on an SDG BelAir. Not that pretty, but man, it's so comfortable.
Been meaning to contact you @gabes cant make my mind up re TD bike. Current fashion for bespoke divide bikes (i.e., Salsa Cutthroat, Moots Baxter) is for more of a gravel bike than a MTB, which kind of makes sense given majority of route (?)