I’m not as young as I used to be although when I get in the gym that concept slips my mind. Then I find myself doing something stupid. Sometimes there is a very valid (kinesiology) reason to do certain exercises in the correct manner. Take for example the squats I was doing that day. I was still warming up and only had 155 lbs on my back/shoulders, but one little glance down and my head shifted which rounded my back and that was it. Form slips for a moment and I’m done for the day and out of the gym for 2 months.

For those who are “out of shape” and want to get back to serious exercise, be careful and go slowly at first. You need to wake some of those muscles up before you can really expect them to perform.

There were a few years when I was more active in aviation where my focus was more on swimming (swim quals) than lifting. For me swimming is something of a low intensity work-out, but my last swim qual was at a higher level. The weight training paid off because unlike previous swim quals with a largish group, this time the group was small and there were no breaks or rest periods between stages. An hour after the test began I was done and it was a really good work-out.

On the subject of low-intensity versus high, I agree with the article at:
Why the fat-burning heart rate zone is a myth - WP
It’s about calories in vs. calories out and if a fitness goal is to lose weight by burning calories through working out, low intensity fat-burning zone work-outs won’t do it unless you have a lot of time. High intensity work-outs OTOH can build muscle and the muscle in turn burns fat.

New Years resolutions are nice, but if it doesn’t fit with your way of life, in the long term it will fail. It’s a process, all year is fitness season.