Good to see another committed person with will-power in this field, glad to have you around.
I'm having a hard time making out a couple of exercises you mentioned but I'll do my best with some advice.
With the upper back 'pull downs', I'm assuming you mean the lat pulldown, fixed pulldown, upper row, mid row machines. For that, I'd strive to get endurance before actual strength. Typically, you'd want to go for 3 sets of 8-10 reps per exercise. 1x20 wont get the job done, you're wasting your time. Sure, you're pushing your muscles for that one set, but it needs some exercise to follow up.
Like I said, as a beginner you should aim for muscular endurance over strength. Lift with the correct form efficiently and slowly and you should be fine. Look to do 8-12 reps of at least 3 sets for the first month. Be sure to add & decrease weight accordingly and don't hesitate to switch your routine every once in awhile, it's for the best.
Good luck, bro.