I believe last year for Dark Side with the way the course was, they did allow people to walk from the Poly. I agree that leaving at 5 would be cutting it close. I think I remember hearing that it was a long walk too.
I know marathon weekend they didn't allow walking but couldn't remember for sure on dark side of course they have changed start locations on that one between 1st and 2nd year.
And yeah when I hear walk over at 5 I think about marathon weekend and the marathon just to get to the start line. Even when I do local races I try to arrive about an hour early. I'm always the type that would rather have more time then less.
Really though the logistics of disney world races just don't allow leaving a hotel half an hour before start. I don't think I'd push it that close at DLR. You're asking for trouble doing that IMO.