I believe the answer is 68.
Explanation:
The sum of the squared legs must give the squared hypotenuse:

But of course we can only accept the positive solution, because
is a length and can't be negative.
I believe a marathoner would have to go approximately 2.9 mph . And or if you are rounding 3.0 mph
15 miles is five times 3 miles.
It will take 40*5=200 minutes to go 15 miles.
200 minutes = 3+20/60 hours = 3hrs 20min