Did it last year.
5am start isn't too much of a big deal if you have travelled from the UK a few days before. The time difference is 10 hours so its like a 3pm start. I didn't do any practicing at all for the early start and didn't suffer for it.
Last year it rained for the first 10k or so and then started to brighten up. It was humid but not excessively so. There was cloud cover so it wasn't as if the sun was beating down all the time. Its not the sort of marathon you want to run all out at - its more one to enjoy as part of a holiday in my opinion in any case. You only have to look at the average times to see that is the story for most of the people taking part.
70% of the race participants are Japanese so the race and expo are oriented fairly heavily towards their needs but that isn't much of a big deal either.
In terms of where to spend time recovering, depends on your time and budget. We decided to stay on Oahu and spent a week or so on the North Shore, renting a condo via vrbo. There is one of the major surf competitions going on around the time of the marathon so you may be lucky enough to catch that.
If you have any more specific questions, let me know.
