I'm not sure many will feel qualified to answer this one. I'd certainly be investing in a visit to a good professional for advice. I can't get away from the feeling that you should probably be deferring your place.
Doing a half marathon off 'no training', as you did in 2009 is really not TOO difficult, if you've been reasonably active in other sports or pastimes. But 26 miles is wholly different. It is not just "more of the same"... it is different. Physiologically, it is different, you use different bodily systems. You have to train. Anyway, with that I'll make a few more comments.
An hour, on sand, running forefoot strike for the first time, was (as you now realise), a bit over-enthusiastic! Probably try midfoot strike next time, see if that's a good compromise. But really... just 2 km to start with.... and very slow. Normally, I'd say that you should warm up with a mile or two of your natural heel-strike running to start with, before introducing a mile or two of the modified style... but I don't know how that would affect your back - so you need to work that out.
Then a programme of stepping up mileages - find one that suits... but don't increase mileages too fast. damage to bones, ligaments and muscles can be cumulative, and not obvious until it's turned into an injury. So don't think that a pain-free run means you can extend beyond the mileage recommended in the training programme. You could do damage.
Be prepared to run-walk the marathon. I don't think you have time to safely train to run the whole marathon non-stop.
Good luck... be sensible. If the advice is that you'll do lasting damage by doing the run... then listen. It's not worth it... even if you're doing it for an emotional charity... no relative or friend would want or expect you to risk your own health to raise money.