You might consider attempting a 24 hour 600k or a 24 hour race first to see if you can do it. I can ride a 300k at about the same pace as your double, but I have never been able to come even close to a 24 hour 600k. Things just really slow down in that second 12 hours, and that's on pavement. The trail would be even harder. I'm attempting a 600k with no sleep stop this weekend, and I'm very confident it will take me a lot longer than 24 hours.