The blast clears that quickly in the atmosphere because the shockwave and debris move at-or-faster-than the speed of sound, so a few hundred meters is tenths of a second or less. You sit inside the mushroom cloud, of course, but the important part is gone quickly.

The Rapatronic camera was used to take these kinds of pictures, and you can see that the actual blast front is around 20 meters across after 1 millisecond (!!) https://en.wikipedia.org/wiki/Rapatronic_camera