Marathon Runner

A marathon runner only runs in windy weather conditions. Magically, the wind in his area is always the same speed, but can have different directions. He knows he can run a marathon in $n$ hours if the wind blows against him. Conversely, he can run a marathon in $m < n$ hours if the wind blows in his direction. How long would it take the runner to complete a marathon in non-windy weather conditions? Provide your answer when $n=20/3$ and $m=4$.