Socks only really get rid of moisture quicker, this moisture helps speed up the formation of blisters. I also have a problem with one blister on just one foot when runs longer than 10Km, this is due to a natural fold in my running shoes rubbing and causing friction. This is the key; any hot spot for fiction needs to be reduces. A lot of runners use lubrication or anti-friction applications such as 'Body Glide'.
The other option is to cover the affected areas with Mole skin (Scholl make a blister kit I believe). This will take the rubbing away from you skin and reduce the chances of blisters, but make sure they are applier without creases or you will still have problems.
There are plenty of web sites offering advice, just experiment and find what's good for you.
Good luck with the marathon
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