You're probably more susceptible to muscle tears after 40, so in your case that would constitute Tears of a Clown. *groan*
Last week my chiropractor, an NPC Master's Champion who hadn't seen me in three months, said "Wow, you really put on some quality size," and I'm a newbie turning fifty! Having not yet reached my peak without muscle memory or a bodybuilding background to rely on there's still the possibility of much improvement so I've yet to really dial it in and become well versed on the subject. Still, some thoughts which are doubly important to those of us who are, ahem, aging:
I lost 70 lbs over 5 months when I first started training doing only steady state cardio and didn't even begin to contemplate high intensity cardio or even yoga until I was under the 200 lb mark given back and knee issues, and then determined that only 10-20 minutes of HIIT on the elliptical 4x per week (coupled with swimming 2x per week) was necessary. The main reason I did HIIT since increasing my metabolic rate in my case is actually counterproductive was that I read that steady state cardio actually reduced heart function because it adapts to the load whereas pushing yourself increases the heart's capacity for stress. Intensity should be adapted to one's particular condition.
I've found that proper form at the expense of poundages is everything. When I look around the gym I see almost everyone using improper form, and not just relying on momentum. For example, most people who use a certain ab machine disregard the instructions on the machine and throw their whole back into the exercise instead of crunching down on their abs; every woman on the buttblaster pushes with her toes instead of her heels and works the lower part of the movement instead of the upper and probably clenches the resting cheek instead of the active one; every woman on the abductor/adductor machines pushes with her feet instead of her knees; everyone on a lat pulldown pulls with their hands primarily instead of their lats first, elbows second and hands third; most benchers press with their hands instead of their pecs and fail to squeeze their pecs at the upper part of the movement; people don't relax their feet when performing leg extensions and leg curls to really feel it in their quads and hams and lots press through their toes instead of heels doing squats and leg presses.
I used to stretch before because of my bad back; I felt I needed to be really limber before lifting to avoid injury, even though conventional wisdom dictates that you should never stretch a cold muscle. Then a Russian gymnast who showed me a few moves taught me that you really should stretch afterwards for that reason. I figured why not stretch before and after and instinctively went for active stretches with ten minutes of elliptical warmup pre-workout and deep static stretches like splits post-workout. As I get older the need to stretch and warm up increases.
We all know that you can't get away with eating what you ate when you were younger. I just try to eat clean with the occasional weekly cheat meal, snack and dessert, reducing carbs later in the day. I don't count calories but I'm sure I'm only taking in about 2500-3000 and if I upped it to a clean 4000-5000 I'd probably add 10-20lbs lean muscle mass but having been fat I'm loath to experiment with that. I believe in moderation, and avoid obvious red flags like added sugar and salt, fried food and junk food. Nutrition is a science which I've yet to master. Speaking of which...
I've yet to master this science and have yet to dial this in as well. I don't believe in using up all your ammo first out of the gate so I started with protein, worked in a pre-workout energy drink with or without creatine (I cycle it) and a during-workout glutamine drink - that plus a multi is it. Existing health issues preclude the option or luxury of testosterone boosters or steroids, yet as a newbie I doubled my low T from 300 to 600 in three years with just diet and exercise. Athletes who have exercised most of their lives such as yourself and find themselves perhaps on a decline having peaked would have greater need of supplementation than I do at this point.