So, for the things I wish I then that I think I understand now:
Leave the bodybuilding stuff to the bodybuilders. The isolation movements, the machines, the split routines, the complexity. My primary sport has always been outdoors, MTB, skiing, or surfing. Workouts that left me too sore to enjoy being outside didn't help.
To borrow from Maffetone, "No pain, no gain equals no brain!"
Little and often over the long haul. Easy and repeatable done near daily works better than intermittent floggings. I wish I knew about approaches like Easy Strength, S&S, ETK, PTTP, etc. years ago.
Park bench vs bus bench training. To everything there is a season. Spend most seasons on the park bench, and be selective when choosing a bus bench program.
Maffetone Method aerobic training. Using the HRM to stay in check. Unfortunately, Maffetone's writing on strength training kept me away, when I really needed some. But if I did then what I thought strength work was, I would have dug myself another hole to jump into.
Yoga, especially Ashtanga vinyasa. I knew this all along, just a failure to execute. To borrow from Dan John, "It worked so well, I stopped doing it."
Kids, learn from my mistakes! Build an aerobic base with MAF, build a strength base with Easy Strength style training, cultivate your chi/qi/prana and go outside to play! Or whatever else it is you do to enjoy life.