I think that the most important thing I've learned about exercise (and it's taken me most of my almost 40 years to internalize it, and I'm still not great at it by any stretch) is that it's probably the longest game of all. If you're fortunate you're going to keep moving until the day you die, and the volume of work you do over that time between now and that day is so far in excess of the volume you can do on any given day that hurting yourself in an attempt to maximize today's volume is just ridiculous. It's easy to use that as an excuse to not do anything, unfortunately, but it's also a good reminder to slow down when your body tells you to.