$20 per mile ran.
So obviously, no money earned on the days you don't run. Pace does not matter in this scenario. You could hobbyjog your way to a paycheck.
Let's go father and say you are LIMITED to 20 miles per day. So you can earn $400 per day, but no more.
But to accept the offer, you aren't allowed to earn money in your free time. So you can't just go knock out 20 miles in the morning, and then go make money the rest of the day.