Totally depends what the event is, IMO. In a huge city road race you won't stand out at all but, in a smaller event like you suggest, it might be problematic.
A pp said they walked the GNR in 3h 24 and there were 16,000 people behind them. Well yes that's all well and good, but the GNR is the biggest road race in the UK.
By comparison, a friend of mine was last in a local half marathon, when she ran it in 2h 35. She didn't give a stuff - but a walker an hour behind that would've really stood out, and would have been on their own, walking on open roads after everyone else had gone.
Your first job I suggest, OP, is to check the finish times for the event from last year and the year before. If there's a few finishers around 6h or more then it'll be worth a go.
I definitely don't recommend running 6 miles and walking the rest, though - you'll burn yourself out early on. Far better to run one mile, walk one, for as long as you can keep it up. Say you ran it v steady in 10 mins, and then walked the next mile in 20 mins, that would work out at the 6.5hrs cut off.
Good luck!