IFQ, don't worry about it so much - after all, it's hard to beat some of the people on here simply because they are World Record holders. There's also no need to get frustrated with zombies, just chillax and try and enjoy it.
Remember, zombies is there to be played however you want - you can try shooting for high rounds, but you don't have to. You can just as easily ignore all the stuff about round 80 or round 90 on here and just play the game the way you want.
And by the way, getting into the 30s is most definitely above average.
My proof: do what you're doing and play with randoms, they are all the proof you need ;)