From reviewing my running activity (a Garmin Coach workout), it appears the execution score is actually calculating as % of time spent in slower half of the target range. So if the workout is to run between 8:00min/mi and 9:00min/mi for 10 minutes, and you run for 5 minutes at 8:29min/mi and 5 minutes at 8:31min/mile, you will get a 50% execution score b/c half of the time you ran below 8:30/mi and half of the time above 8:30/mi. This doesn't make sense. I'm on 11.28.