How do I make my sourdough bread dough stretchy and not have it tare?

I've been trying to make chewy sourdough bread and it's not going as planned. I read somewhere that if you mix and knead the bread a lot and avoid adding a lot of sugar and use a lot of water that should aid gluten development to make the dough chewy. Also I read that the dough should be stretchy and if you pull two pieces apart it's supposed to be transparent. Well, I don't have anything anywhere near that.

My starter was super fluffy. I took 2 cups of starter, added about 1 cup of water with 1 teaspoon of salt and 1 tea spoon of brown sugar already dissolved. Then I kept adding flour and mixing in one direction until it didn't mix with a spoon anymore. Then I kneaded it for about 1/2 hour to 45 minutes. I tried to avoid adding too much flour to keep the water percentage up, but that's hard to do when you are kneading on a wooden board and the dough keeps sticking. I finally got it to where there was an even layer of dough stuck to the pen so the piece I was kneading didn't stick to it too much anymore, but the dough still refuses to be stretchy. I pull on a piece and it tares with a rugged edge. That's after 1/2-3/4 hour of non-stop kneading. Finally I gave up because it wasn't getting better.

I let it rise for 1 1/2 hours between a heating pad and covered with electric blanket on top. Then I kneaded it again. At first it was slightly more stretchy, but the more I kneaded it the less stretchy it got. I didn't add a lot of flour at this stage either so it kept sticking to the board. Now it's back to the original non-stretchy, tearing consistency.

Can anyone tell me what I'm doing wrong?