I'm considering commuting...I'm 5'4" and 125 lbs. Already in really good shape (run 10 miles everyday). I used to bike 30 miles a day before I got a job (instead of running), when I did that I gained a little weight in the form of leg muscle and upper body fat.

In my experience, biking builds leg muscle more effectively than running, but does not burn fat and tone your core nearly as effectively.