>Cross Training>Weight training and injury prevention for runners
I probably wouldn't think of it that way. I've never heard of the 80% thing. Just stick to compound, multi-joint exercises like squats and deadlifts and you'll work all your muscle. For upper body, push-ups, bench press, pull-ups, military presses are the ticket. Don't overthink it. Keep it simple and you'll get the best results in the shortest amount of time.
Champions are made when no one is watching