I think the most useful exercise is running because running is very simple.
So running is the most useful exercise.
Here's your corrected composition, Jimmy. Please take a look at the corrections below. See you later! 
What is the most useful exercise you can think of? Explain.
I think the most useful exercise is running because running is very simple. So running is the most useful exercise.
>>> I think running is the most useful exercise because it is very simple.
>>> OR: Running is the simplest and most effective form of exercise.