Yup, do it 'till it's a habit. The amount of time it takes to become one will depend on how motivated you are to make it one. I came to grad school determined to ride to class, so once my parents got me a bike, it was easy. Now I ride back and forth about ten times a week. My round trip is less than four miles, though, so at 40 miles per week, it's considerably less than what you're doing. If you're comfortable with music, I recommend a set of quality
earphones and an iPod filled with albums you like. That got me through walking everywhere in undergrad, and it's getting me through biking to class in grad school.