I'm not talking about being great at writing shorts vs being mediocre at writing features. I'm saying that if you can't write 4 scenes well, the chances are very unlikely that you can write 40 well. If you can't write 35 lines of dialogue well, what makes you think you can write 700??
Too many novices get into screenwriting for the money, or as a means to direct, but If you're heart isn't in writing chances are you aren't very good, so stick to writing for television in that case. If you have a love for screenwriting, you should have some outlines, unfinished scripts, short stories etc laying around....make something real if you don't have anything produced/sold under your belt, even if you have to produce it yourself......but writers write....they are the only people who don't have to wait for there to be a greenlight from a studio
"Cling to your work, and you will create nothing that indures" -Lao Tszu