Blisters. They are almost inevitable. When I started running they were just a mild inconvenience as I never ran long enough for them to be a problem but as the kilometres started piling up they can turn into a real unpleasant issue.
After my first marathon, I lost four toe nails. After my second, I didn’t even get a blister.
The difference was properly fitted trainers. If you thinking about running a marathon, get an assessment in the sport shop where they will recommend the right shoes for you. Trust me, it is worth it.