Marathon Pre-Planning?

Walking to and from school twice in a day, which happens most weekdays, is about an eight mile committment. I could rent a bike for the semester, but the way I see it, this is a decent way to build some leg strength if I decide to start any serious training for next fall's Chicago Marathon once I get home this summer. That, or work off any Guinness that happens to fall into a pint class in front of me.