Let me start with a short intro. I'm 21, 6ft tall and weigh 63kg (~138lbs). I'd much rather go to the gym and do a load of cardio than weights. And until recently i'd run (~5miles) then do 10 minutes of any random set of weights. I've decided however to relax the cardio slightly and try and bulk up a bit. Hence I found this great plan which I started today.
I'm kinda fit already so I decided to start at level 5 and managed 18 reps. My upperbody strength is a bit out of proportion compared to my core/legs so the squats really hit me hard and held me back.
It's funny, those of us on the heavier side of things (I started at 301lbs) have strong legs, weak upper body. Those on the lighter side as yourself (I'm only 3" taller) are the reverse.
Physiologically it makes sense, but sometimes the obvious just needs to be said. Welcome!