You walked 8/10 (or 4/5) of a mile.
To find this, you must first make sure both fractions have a common denominator. To do so, multiply 2/5 by 2/2 to get 4/10.
Now, add both fractions. 4/10 + 4/10 = 8/10. To simplify this, simply divide 8/10 by 2/2 to get 4/5.
Thus meaning your answer is 8/10 (or 4/5) of a mile.
I hope this helps!