Forgive me for originally thinking this was a completely hairbrain idea. Your second posting sounds a lot more reasonable and more like you've really thought this over. Your current physical conditioning and outdoor experience should allow you to get into "cycling shape" fairly quickly.
As to a source of reasonably priced bikes (I assume the Walmart suggestion was a joke, I certainly hope it was!
) do you have a local Craig's List? That can be a source of very good bikes at low cost. Also, post WTB (want to buy) notices in local bike shops specifying what you are looking for. Most have bulletin boards for this type of posting.
I would avoid mountain bikes unless you can find decent quality rigid fork/hardtail (i.e. no suspension) model and put slick tires on them. They will have a weight disadvantage but will have appropriately low gearing and most have rack mounting eyelets.
Otherwise road bikes are preferable for their reduced weight but pure racing/sports bikes will have too-high gearing and lack mounting eyelets for racks. The suggestion for a cyclocross bike is a good one as they are rugged and can be regeared rather easily. Obviously, a real touring bike would be ideal but used ones are rare.
Finally, no matter what, get bikes that FIT. Bikes are like shoes, you want the right size. If you have a knowledgeable friend or if there is a bicycle club at your school or in town, by all means get their help with choosing and setting up what ever bikes you buy. Then ride, ride, ride. And enjoy.