How it is supposed to works? I had a 5km run (pace 3:47) and, as you can see, the only improvements is in the "aerobic" level. That wasn't an effortless run at all. Why the anaerobic level didn't improve?
see forums.garmin.com/.../anaerobic-benefit-query
The type of steady state run you had is totally unsuitable to generate anaerobic benefit (at least in Garmin's mind).
I was given excellent advice in the above thread linked by 4684381, strongly suggest you read it. Good luck.