Actually DVDs contain on anywhere from 4GiB to 9+GiB of mainline video... The whole reason for dual-layer discs was to get around the limitations imposed by the ~4.5GiB max of a single layer. The "base" read rate of a DVD (1x) is 1.32MiB/sec, which puts a 2-hour movie at peak bandwidth (hopefully because they actually maximized video quality rather than just being lazy with the encode) at 9.5GiB, just under the absolute max. Account for menus, special features, and average encoding levels, let's go with 6GiB.
OTOH, 2mph is rather low, with one source (http://answers.yahoo.com/question/index?qid=1005112200029) claiming studies put the average healthy male at 3.4mph. I'd also estimate there to be maybe a 50% functional walking time (eating, blisters, etc.).
And as others pointed out, on a serial line there are actually *10* transmitted symbols in a byte because of the start and stop framing. 11 or 12 if you add a parity bit and double-stop, but let's stick with 10.
"Furthest" redbox would likely be something like from Portland Maine to Los Angeles for example, which google puts at about 3100 miles.
3100mi / 3.4mph = ~911.75hr / 50% *roundtrip = 3647hrs
6GiB * 10bits/byte = 6442450944 symbols
6442450944 sym / 3647hrs = 490.69 baud
If one were to drive, as per Google it'd take ~46hrs with traffic, and if you trade off drivers. For that roundtrip I get about 19452 baud. Let's say 19.2Kbaud to leave time for chinese fire drills.