Please help– foot pain

Almost two weeks ago I participated in a marathon walk on hilly terrain. I felt fine throughout however, I woke up the following morning with severe foot pain. The pain is on the outside of my right foot, between the heel and the pinky toe. For the first few days I could hardly walk on it and the pain was excruitiating. However, when I was seated, my foot did not hurt at all… I can also drive with it. The pain is strictly when walking. It doesn’t hurt at all when I first wake up, but gets worse throughout the day to the point where I feel my foot is tearing in half. I went to an urgent care clinic and the x-rays came out negative. The doctor gave me crutches and told me to wait a week. Since I’ve been off of the foot, it feels much better however, if I walk without the crutches the pain begins to return after a while.