John runs at 6 miles per hour for 0.5 hours, for a distance of 6*0.5=<<6*0.5=3>>3 miles.
Then John runs at 4 miles per hour for 0.5 hours, for a distance of 4*0.5=<<4*0.5=2>>2 miles.
In total, John runs 3+2=<<3+2=5>>5 miles.
#### 5