Outer forefoot pain

i’m training for my first ironman. just started running over 18 miles, and in both times that i ran this distance
i experienced pain in the outer half of one of my forefeet. this pain lasts about 4 days.
first time i ran with with older shoes (not too old) and the last time i tried a new shoe and had the same pain.
anyone knows what could cause that?
i’v trained for marathons before and i’ve had knee pains but never forefoot problems…

are you running on the road? It could be from the camber of the road, especially if it’s on the foot that’s the side of the road you run on. Ie, left foot if you run on the left side of the road. Those muscles get worked a little bit differently with the tilt in the road and as your form falls apart as your run longer then they get overworked.