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<< Also, be sure to stretch before and after exercise to prevent stiffening, minimize your risk of injury, and enhance flexibility as well.  It's a PITA but well worth it if you plan on doing this more than one week.  I find that the pre-ride stretch improves performance as well. >>
OK, cool, thx for the advice. Do you think, if I make from now on, everyday a 20km ride on my bike, I will be a little bit more fit on next Saturday? >>
Probably not...a lot of getting stronger is recovery >>
Crap, should have started a little bit earlier. Oh well.... it will help a little bit I guess.... 
Maybe I can bike to work everyday.... hhmmmm