I believe that riding centuries is as much about time in the saddle as it is about fitness. Although I've not tried to do a tour without time limits. My century rides have mostly been supported rides with time limits. The other one I have done was unsupported and I was using it a s a pre STP test to see if I was ready for a one day double century, I was.
Only you know if you can do a 300 mile weekend or is it a 200 mile weekend? Which 300 miles also matters. If it is rolling hills the the challenge is even greater. Good luck!
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Sir Mark, Knight of Sufferlandria