i've done several hilly centuries (and over 100 miles) rides already this "season".... 3 VERY hilly ones in 3 weeks! then a "rest" with a less hilly century... then another hilly one.... etc etc etc!

if you are conditioned (speed means nothing IMO... i'm slow and i still do the harder, longer rides! just takes me longer!) then you can do centuries a week (or day) apart if you want!