My guess is that most people don't run very long distances (eg bob graham sort of distance) at marathon pace, which means that the 40 calorie rule goes out of the window. If your excercise intensity is lower then I imagine you can take on more food.
In a race situation where you're running at the threshold of what you can sustain for eg 3 hours you'll probably only be capable of taking on board smaller amounts and then predominantly carbohydrates. On a long day out you can consume all sorts of goodies and will probably need to, as a monotonous diet of carbs from gels, bars and energy drinks is likely to upset your stomach. One of my favourites when I lived in Sheffield used to be to run to the Grindleford cafe and have one of their legendary chip butties with a pint of tea and then run back again - more like 1040 calories than 40 but as long as it was 'steady away' from the cafe I never had any stomach problems.
I don't know what someone racing as hard as they could for much longer distances than the marathon would typicaly eat - interesting to hear from others.