The average person could not reach a speed of 12 mph even for 1 second.
Let's assume you sample 100 random strangers who are ages 15-50. Even in that age bracket, half of them are not even going to be able to achieve that speed, probably well less than half.
Now instead of average person, if you say average hobby joggers and runners... Then you might be able to give an actual distance to your answer because probably half or more can reach 12mph
Source: myself