I was really concerned about this due to the fact that the first 100 miles are usually somewhat of a struggle for me. The only bright spot was that I seemed to recover at 100 miles. Turns out this was a pattern throughout my SR series, I would feel like crap from 40 miles to 90 miles, and then I'd feel pretty good the rest of the ride. Of course, everyone is different. But I do think that most people that can do a 200k can do a 300k without issue. I still feel pretty hammered after a 200k, it's strange.