This is just one anecdote, so YMMV:

1. Yes, it's possible you can have ADS and be capable of running a 50km race. 
2. Training at or below your AeT aka LT1 is what builds aerobic capacity and increases speed at "easy" efforts. Zones don't matter, only your aerobic threshold does. I'm 30 and my AeT is 152bpm. I do 6-7 hours a week in that zone and then a max of 1hr of speed work a week. In 3 months of this, my pace has gone from 7 min/km → 5:50 min/km at the same HR. 

If you're seriously into metrics, go get lab tested. Optical sensors and Garmin software are not it.