Hi threepercenter, you sound like you are in exactly the same predicament as me (without the hormone issue however). I am 47 and although I have experimented with weight training since my teens, it has been on/off and never properly serious. The biggest mistake I made was when I was in my late 20s, and having bought some free-weights and equipment which I used for around 6-months, I then joined a local gym (which I went to 3 times a week for a number of years) but could never (or didn’t feel I could) get onto the muscle building machines. I thought I was an ectomorph and that there was no point in trying to get big because it was not going to happen.
I stopped weight training altogether in my mid-30s until decided 2-years ago that it was now or never - should I give it one final push to get the physique I had wanted since I was teen. I decided, yes - do it as an experiment if nothing else, keep a log and if it works pass on the knowledge, maybe even write a book! However, based on previous experience, a gym was not for me, too many posers, too many sweaty betties, and not enough equipment to allow me to do the workout I wanted, in the order I wanted, in the time I wanted. I went and bought a multi-gym, cage, leg press, bench and numerous free weights (over 500lbs), barbell and dumbell handles (all from ebay so not a huge cost). I started with whole body workouts, and then splits into upper/lower body, and now most weeks I do a 4-day split (legs/chest&back/shoulders/arms). I do abs and calves every workout (except leg day!). I do not do any real cardio. Over the 2-years I have definitely put on some muscle, lost some fat (mostly around the middle), but, now I want real mass!
I am also 5’9" and weigh-in at around 154 at the moment - reasonable definition but no real size. Like you I think 170-175 would be an achievable target for some natural growth that we should still be capable of even at our age. I have read some of the posts made by other over 40s and they are quite simply inspiring and has made me more determined than ever.
I am new to this site having previously used workouts on bodybuilding.com and I must say the routines on here seem a mile (in a good way) from those on the aforementioned site, and appear to utilise the most modern training methods and thinking. Like you the one that caught my eye was 5-3-1, and I am just about set to spend my next few workouts dialing into my 1RM on the core lifts, and would be interested to get your update on how you are getting on with it. I know you only posted 5-days ago, but, are you still noticing real differences in your physique? Would you mind posting your routine on here? What are you doing as regards calorie intake and supplements?
Sorry for the long post, but, sometimes just simply putting it down on “paper” albeit virtual paper, makes it more real because you are sharing your thoughts/experiences with like minded people.