I suspect Kerry's full of crap. Nobody just runs a marathon. It takes dedication...running regularly, practicing a certain way. And nobody forgets. If you run a marathon ONCE it will be especially "seared" in your memory, it's that tough and therefore that much of an accomplishment. Typically, runners are very proud of it, never forget their time let alone the date...my impression is that runners who are advanced enough to tackle a marathon tend to get fixated on details: time, pace, training methods, shoes, etc.
New Balance 320's, trained at 10 mi per day all year long and 15 mi/day 60 days in advance, carbohydrate loading all the time... You don't just up and decide to run 26.25 miles a day or two in advance.