Increasing mileage

[sadieme]

I have just starting cycling and am trying to figure out how much and when to increase my mileage. So far, I have been out on the bike 3 times.

12 miles
14 miles
20 miles

The first two were 2 days apart. Then another week went by before I did the 20 mile ride. All of them kicked my butt. I was totally spent by the time I was done.

I am wondering if there is a general rule of thumb about increasing mileage or if I just go by feel? I am also trying to do a short run at least one day a week, maybe two. Is it good to increase mileage by the week, or always just go as far as I can on a given day? Is it like running where you do one long run a week and vary the rest of the rides?

I want to increase my fitness as well as lose some weight, if that helps.

Any thoughts or suggestions would be appreciated.