I started out doing one or two miles to get used to the feel of the road bike when I first got her (I hadn't been on a bike since I was ten). I bumped it up to five or ten after a while, and then have been trying to move up to fifteen or twenty. Then I got the flu, and now doing three miles has me tired. Do what feels right. There's no "should" here. Well, I know I should ride more before winter sets in, but alas, Cleveland's weather, terrible pavement, and school conspire against me...okay, mostly school.