Ok, I did a 40 mile RT for awhile and as the time went on I got faster. I think I got it down to just over 2 hours. It will take you about a year to hit anywhere near what you will ultimately be able to reach in terms of speeds. With the kind of mileage you are looking at eventually moving into I would scratch the wally world bikes right off the list. You are going to be chalking 6000+ miles a year on this thing and you need something that will be dependable. I would look at something more in the range of $600-800 new or $200-300 used. Your needs are completely out of what the x-mart bikes are designed for. If you have heartburn on the price add up $50 a week for the next year plus $30 a month Gym membership (since you will be getting your excersize during travel). That adds up to quite a sum. Your plan to increment the distance is a good one. A road bike with 25c or 28c size tires would likely be able to handle some pretty rough pavement and that is what I would recommend for the distances you will be travelling.

(My current commute is 28 miles RT from the area I park at)