If you are 20' behind a reheated F-4 Phantom then you are standing in a flow of air that is
a) still supersonic (look at the shockwaves in a Phantom-II's exhaust during a night takeoff)
b) still rather warm (800C or more)
I've been about 100 yards behind a Vulcan starting to taxi and even the minimal amount of power used to start moving was enough to make me crouch down so I didn't get blown over, and I also turned away so I didn't get a facefull of soot...
