The more I read of the hadd techniques, the more confidence I have that you need to be slowing down, not speeding up.
A lot depends on how long your runs have been. If you've done 20 miles at 9 minutes... then it shows ability. If it's 10 miles, then it's unproven.
I'd suggest you can still get to 4 hours, if you've been getting plenty of miles in - even if you have been going too fast. I think 4 or 5 weeks with a lot of slow running.. at the pace cougie suggests... could save the day. You haven't got time to waste a single run at a pace faster than 9.5 minutes.. and probably better having every mile slower than 10. Good luck.