You're obviously already capable of doing 50. 75 may not be comfortable, but, chances are that with your current fitness you'll be able to complete it. 100? I would suggest more time than you have available between now and the event for preperation. Idealy you add 10 or so miles to your long ride each weekend up to 90 or 100 then taper off for a couple weeks before the century. The 75% rule works fine when you're consistantly doing that distance or close to it. Not so great when you're on an extremely excellerated program to reach that total withing a week of the target event.
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