My advice would be to not worry about miles or time, they are somewhat useful reference points but the real reference is your body and what it tells you.
Start out nice and easy when the pain starts take a rest then do it again the next day.
In a surprisingly short time you will be able to spend all day riding with minimal discomfort.
30 or 40 miles on my Giant Cypress is an easy ride for me now, I plan on doing at least one century ride on it this year.