I finished a 200 and two 400s this spring and have my first 600 scheduled for this weekend. For the 600 the wind might pose a problem into Sunday depending how quickly it ramps up to 60 km/h.
The second 400 confirmed my observation from a few seasons ago that riding with too little effort can cause significant problems. My average HR for that ride was one of the lowest I've had on any ride. When dialing the effort way back I couldn't use my regular 90 - 100 cadence since it feels weird pedaling that fast with so little resistance. So I had dropped my cadence to 60 - 70 just to keep some weight on the legs. Even so weight seems to shift from the legs to the saddle, so I had to stand much more often. In general going slow just leave me feeling lethargic. And late at night with a much lower HR my wakefulness was significantly reduced. After the ride ended I picked up my pace for the ride back home and woke right up.