This was another nugget of wisdom from my junior rowing days. This philosophy, and a whole lot of peer support kept me showing up to practice even when I didn't want to.
I'm not in a no-pain-no-gain camp, and I fundamentally believe that you _can_ overtrain and hurt yourself. Simultaneously however, I also recognize that there is (for me at least) a fair amount of fantom "I'm too tired", or "I don't wanna" energy in coming to train every day on my own. Pushing through this (smartly) yields pretty good results.
I am a big fan of having someone else manage your training. If you can't do that, program in breaks, and active recovery days, and be wise about when you feel overly sore, or overtaxed.
There are probably some indicators of overtraining that you can legitimately pay attention to, and it's worth googling around for them. However I also feel like experienced athletes (even over-the-hill, fat, old ones), should have a pretty good sense of what is a sane workout program, and what is overreaching. The goal here isn't to hit it like a marine at bootcamp for 2 months, and then never go to the gym again, the goal is to be able to train on a regular basis, for life.
All of that being said, if you have the means to engage a trainer, or go to a regular, structured class, I highly recommend it.
