I finished up my first Super Randonneur series last weekend with the Santa Cruz Randonneurs Surf City 600k. I did the same ride last year so I knew what I was getting into and that it would be tough. I DNF'd the SFR Fort Bragg 600k a couple weeks due to calf issues and unrelenting nausea, so I needed this one to wrap up the series and qualify for PBP.
https://ridewithgps.com/routes/29720174
The hard part of the Santa Cruz 600 is that there's quite a bit of climbing in the first 100 miles, then a strong and steady head wind for a good portion of the last 200k. For the nausea, I changed my diet to be really carb heavy and light in protein/fat and it worked really well, I had no problems for the whole ride. Otherwise, it was in the upper 40's / low 50's and raining for several hours on the second day, and I think the rain combined with the head wind led to about a 50% DNF rate (there were only 13 or so people on the 600k and about the same number on a concurrent 400k on the same roads). For me it's mostly a mental battle, being at a control and knowing that there are ~60 miles ahead of almost non-stop, strong head wind that will mean an average speed of probably 10-13mph, with rain on at least part of it...but I'm really glad now to have gotten through it. I rarely get muscle soreness from cycling, but after this one I'm pretty sore, like I was doing squats and lunges!