Some of you guys are doing the OP a disservice by just saying "do more miles". What kind of shape is he/she in? What kind of condition is he/she in at the end of the 8 miles? Done? Or ready for more? Why are they stopping at 8?
And I know there will be some chafing at this but WTF. Yeah I'm doing small mileage now to get in shape with the goal of more as I become conditioned to riding,hell physical exertion in general. I don't even keep track of mileage but have a general idea of it. If you're in such good shape that you cannot recall the feeling of your heart at maximum effort congratulations. I'm just not there yet.