Indeed - great bit of road. Never done the 10, done the 25 and the 50 but as you say it’s to far away IMO for a 20min race. That said at least for the 10 you don’t have to get up at 5am like you do if you ride the 25 or 50 as they are always Sunday morning 
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FWIW - relatively flat courses here in Mid-Atlantic USA, I was typically good for low to mid 57 minutes 40km on 225-230w average power. Calibrated SRm power meter. Body weights between 132-140 pounds and my aero tested well at the A2 tunnel (apologies I forget the CdA numbers or would share).
Had a goal to get into the 55 min range, but when I had the potential fitness (240-245w FTP), never managed to catch a good condition day. It’s always a chance with wind or heat or both when the 40km events come around in June/July/August.
In the end, I had good aero properties, and extracted most everything I could from position and equipment, but never had enough power to get really fast for 40km. C’est la vie, it’s always fun and a good test.
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16 or 17 years ago I managed a 58 min 40 k on the TT course in Moriarty NM. The equipment was a Cervelo P2 with a covered wheel “disk” in the back, a tri-spoke in the front, SRM power meter. Height 5’ 10 in, weight 156. Standard lycra skin suit, Aero helmet. Avg power 220 watts. Yes, you can buy speed.
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