Wow, 17 hours is 10 hours to spare! That's awesome. A 300k would be 52 miles further than the route I just did, so I feel it would be possible for me to do. Pending on the route, (good) weather and (lack of) mechanical issues, I guess I could assume a 300k would take me 15-16 hours. I am FAR away from a 17 hour 400k, lol.
Good luck on your 600k! I will be following FB for updates from that group.