When one is told that someone is running a marathon, one should react with awe and lots of praise/encouragement - so it should be regarded as cool, suburban mom or not.

Note that in some demographics the marathon has been superseded by the triathlon, preferably an Iron Man, in coolness.

(I am not questioning the physical achievement - I did a couple triathlons in my early thirties and sometimes I do a mini-one at the gym given the time and energy - but for regular people to do them isn't that unusual but to advertise it far and wide is a sign of status seeking imo.)