Haven't played in a bit but i've completed ~230 levels on SMB including Cotton Valley & "The Kid" Warp zone, although by all means most Dark World levels and "-" levels are harder.
I spent ~4 hours getting "The Kid". I was playing non-stop and my gf was just doing thing around the house. The last level took me the longest, probably because my thumbs were sore and my eyes were burnt from being focused for so long. My gf was able to watch Space Jam all the while I was constantly dieing from the flying spikes, eventually the movie was over, she was passed out and I was getting frustrated. I said to myself "Alright one last go", and then I made it. That feeling was absolutely awesome, it's like I'd been holding my breath for the past 4 hours and finally got to breath.
Wow,I only just unlocked Super Meat World (Mid-January patch for PC) with 20 bandages and before that I'm stuck on the third level of The End (Keyboard control is a pain in both the metaphorical and literal means,CARPAL TUNNEL LIMBO MAMBO!) Give yourself a cookie good sir!
If you can pick up a wired 360 controller, they work very well for most PC games and especially SMB. You can probably find them pretty cheap.
I didn't think it would cost any less than a normal one,still I'm planning of getting a 360 controller for my laptop,otherwise the dark world is a pipe dream...