In reply to Bjartur i Sumarhus:
> ...the question is this ... If I had managed to get in decent training and was achieving my training goals up to a few weeks before the marathon, what sort of damage could a luxury holiday in Thailand do two weeks beforehand? ... I expect I will be drinking a bit and laying around eating in the sunshine ... in a luxury villa with full works ...
Speaking as a professional: that sounds like terrible preparation, and I strongly advise you not to do it.
However, since I'm a friend, I can help: I hereby volunteer to take that luxury, two-weeks, all-inclusive, eating-and-drinking holiday in the sun on your behalf. And you can stay here to complete your training.
You should then be in great shape. Good luck in the race, feller!