My coworkers at work think that a true distance runner runs a marathon regardless of their weekly marathon or actual race distance. I run cross country and distance in track for my school, not dreaming of a marathon.
One co-worker will routinely ask this other coworker who HAS run a marathon when he once was going through a hobby jogger phase (he no longer runs) about marathon training. She asked what his longest premarathon run was and he was said 12 miles, but his weekly mileage was in the 30s.
I remember reading somewhere that without the right training running a marathon can actually be bad for your heart, like a mild heart attack or something. Makes sense, 26.2 miles is serious, but how much mileage should a person be at to be in the physical shape where that isn't the case? I tried to explain to my co-worker most professional marathoners run well over 100 miles a week and she just discounted it "since they're professional" instead of understanding that a marathon is something that takes a huge base to build up to..