In reply to Mikelat:
I have done this, though admittedly it was only 1 week in Devon not 2 weeks in Asia! It also happened to be the Brighton Marathon I was coming back to.
On the holiday I tried to go running to keep the legs moving but it never felt great, which just made me worry more about the marathon. One of the runs along the coast path was lovely, but my legs weren't half complaining about it!
I ate out and about and more stodgy fare than usual (I was on hols!). I also drank a reasonable amount of alcohol. I didn't sleep as well due to staying up a bit later and perhaps the alcohol and different surroundings. We were out every day either cycling or walking around attractions.
On marathon day (the day after a 5 hour drive to get home) I felt reasonably OK on the start line, but after about 3-4 miles I felt quite sluggish and tired and it turned into a bit of a slog. I didn't get the time I was hoping for. This could have happened anyway, but I'm fairly sure I was a bit heavier and more tired than I would have been. So, I got round, but not very fast.
My advice: give it a go, make sure you've done all the training before you go on hols and enjoy yourself (within reason). Try and run when you can to keep the legs moving. Do not expect to get a great time - but you will be able to finish if you've done the training before the holiday. If this is your first marathon, there's loads of other stuff that can go wrong anyway (weather, nutrition/hydration, chafing etc.). If you enjoy it, you can always enter another one which isn't just after a holiday.