Sorry to be slow responding.
Not sure where you read 80%. Here is the recovery run instruction copied from the first ultra plan. It says 90%. That might allow you to run a bit more of these.
Successful training requires modulation, which means workouts must be completed at differing intensities and durations to elicit the desired response in your body. Recovery workouts are often overlooked because they feel easy, but don’t be fooled; they are as important as any other part of a training regimen. As an athlete gets stronger, recovery workouts become essential to counterbalance the strain placed on the body by more challenging efforts.
Recovery workouts should be performed in Zone 1 or below. As a rule of thumb, the top of Zone 1 can be calculated as 10% below your aerobic threshold (AeT). Take your AeT and multiply by 0.9 to find the top of your Zone 1.
I hope this helps,
Scott