When I finish this race, I will have run a total of five miles.
When I finish this race, I will have run a total of five miles.
Why do we use present simple in the first sentence Although we're talking about action in the future

Question

When I finish this race, I will have run a total of five miles.
When I finish this race, I will have run a total of five miles. 
Why do we use present simple in the first sentence Although we're talking about action in the future

Answer

So we don't confuse the two clauses.

"When I finish this race" is not the main action. It just gives us a time-reference for the main clause: "I will have run a total of five miles."

With future perfect (will have run), you definitely need a time to refer to.