mowzer - as racers are stripped down trainers I don't find they vary as greatly as heavier trainers. Certainly all the ones I have tried have fitted and performed well without extensive searches (unlike normal trainers).
Your best bet is to try a few pairs on and whichever fits best you go with. The last two pairs I have bought off the internet and they were fine. I shopped by weight, available size (12) and price - I also took into consideration the pronation characteristics (but sometimes ignored them anyway).
I have adidas adizero manas and hear a lot of good things about the rest of the adizero range.
You can certainly do marathons in them although as with every new thing you will first have to establish a bit of injury free training history while using them. My suggestion would be to go for some lightweight trainers (as opposed to really stripped down flats) and gradually integrate them into your training. If you have no problems you can take it a step further with racing flats. At the very least your end goal should be to do all your quality sessions in flats where you can really use the foot's anatomy to your advantage.
There's nothing wrong with a variety of different shoes for different purposes - e.g. racing flats for races, spikes for track work, xc spikes for cross country work, barefoot on terd-free cricket pitches, lightweight trainers for easy distance, big thick heavy ones for long runs etc. But the overall direction should definitely be towards the 'less is more' end of the scale.
As a disclaimer - I think running shop 'gait analysis' is next to useless. Others feel (or more likely have been brainwashed) differently. So bear this in mind with regards to my personal buying comments.
Edited: 18/02/2010 at 23:46