If later on you want more speed and distance I highly recommend an EBike kit. The point is NOT to push you along without any effort, but to assist intelligently and make it so you don't have to sweat while biking around.
http://www.nusun.com/E-Bike-Kit-p/nn%201217f.htm
There are lots of kits like that that will work. The one I got is not on the market anymore, otherwise I would recommend it. All said and done you can get a cheap bike, add a kit on, and have a nice electrically assisted bike for under $500. With peddling I get a range of about 15-20 miles without breaking much of a sweat and averaging 25 MPH.
Beyond that the kit becomes dead weight, but even then the whole kit only adds 15lbs.