James - in terms of timescales I agree with what everybody else has said - defer, or if you really can't bear the thought of doing that then work out a realistic run/walk plan that will get you round without breaking you.
However, I have, over the past few months, adopted some of the Chi running techniques and they have made a big difference. BUT it has taken months, not weeks to embed the changes and I'm still not there yet.
Get yourself a copy of Chi Marathon by Danny Dreyer - it's a bit tree huggy in style for my tastes but it very clearly teaches you how to get your posture right, which is the biggest thing. It takes lots of practice in front of the mirror and a LOT of correcting when you're running but for me it has been worth it.
It's worth a go, but realistically it's going to help your next marathon, not this one.
Good luck 