Yup, Joe called it. Classic mistake of overtraining.
9 out of 10 gym noobs think more is better. If 3x is good, then 5x is better. And then I'll go 6x or 7x! Hell, when I really get cooking I'll go 14x, just like Arnold!
Back off. Your metabolism runs way hotter than those big lumbering guys in there 7x for 2 hours at a time.
Work out every other day max. Work out hard, intense, don't sit and rest and gab like the other 90% of people you see, and NEVER work out more than an hour. Think of an hour as your "Cinderella turns into a pumpkin" time -- but instead of a pumpkin, your body will be switching into catabolic mode. (If you don't know what that means, your first homework assignment is to research "catabolism" on the innerwebz -- you need to understand this completely as one of your bodybuilding prereqs).
Focus as much on getting enough sleep (where your muscle actually repairs and grows) and proper diet as you do when working out (again, 9 out of 10 noobs just focus on working out, which leaves 2/3rds of the equation at zero), and you'll see results. If you don't understand the mechanics of how muscle breaks down during a workout, and how your body builds it back up while you're sleeping, you're just lost, wandering in the dark. Learning this is homework assignment number 2 -- all the answers are on the innerwebz.
o Don't overtrain.
o Pay attention to your diet (this is a long complicated thing, so you'll need to do your homework).
o Get 8 hours sleep minimum -- 9 or 10 if you can swing it.
o And be consistent, and patient. 9 out of 10 noobs have incredibly unrealistic expectations about how fast they can change their bodies. This is a lifetime commitment, not a 4 week project.
Sorry if I seem to be patronizing, but after 10 years of watching forums like this, it's the same freakin thing over and over again, and I've become an old cranky guy. Normally I just look away, but the fact that you quit smoking shows some courage and determination, and based on that (I know how hard that can be), I think you can do this if you never stop trying to learn.
Your brain is your best weapon against your body's extremely skillful ways of staying lean. Your body is an incredibly savvy and persistent adversary. You need to stay smart to win. If you do, then nobody can call you a noob!
PS Note that I didn't suggest a single exercise, or talk about the best split, or say anything about what you do in the gym? It's because that's the least important and easiest thing to tackle -- which is why nearly everyone obsesses about exercises. The real challenges of gaining muscle for naturally lean guys are much harder and more important than what exercise to do.
PPS Note also how cranky I seem? I haven't had enough cookies. This is my excuse.