Meaning that at 20m from the center of the explosion (the fireball was 20m diameter, or 10m radius) it should already be at 1/4 the intensity, already less than the surface of the sun. At 7 miles or 11200m, you're talking 1/(1120^2) or 7.97e-7 of the original intensity - effectively zero - if I've done that math right.
Let's not forget, apart from distance, the fireball is expanding in all directions. At 1" from the center, you're exposed to half the fireball. At 1 mile from the center, you're exposed to a tiny spherical arc of the fireball.
The energy is going in every direction, not just at you. With distance, you will receive a smaller portion of whatever energy made it 7 miles out.