Running

Ok next year I wan to do 5k below 20 min. My plan is to simply start with 1k sub 4 min and then just make extend the distance but by bit until I reach my goal. Does that approach sound good. Will mix it with some different runs and intervals.

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Yeah! That sounds much more efficient😄 will give it a try!

mmm strange approach. it might work for you but I've never heard anyone training like this...
I would suggest doing some reps 3x1000 or 4x1000 at race pace with some static rest in between or 500m jog.

Interval training, tempo runs and prep races should help.

Best way to improve as a runner is to be consistent about running. Your approach sounds fine but my worry would be burning out because that’s a hard effort quite often. 

Even running 5 days a week for a few months without any effort at all might get you to sub 20 depending on how close you are now.

All about consistency!