I always operated with the formula - If I can do 75 miles with comfort in the same conditions (weather and terrain), I can do the 100 miler too. At 63 miles, you are real close, and I think az-cyclist makes a good point. An 80 miler would be a good test.
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A conclusion is the place where you got tired of thinking. - S. Wright
Favorite rides in the stable: Indy Fab CJ Ti - Colnago MXL - S-Works Roubaix - Habanero Team Issue - Jamis Eclipse carbon/831