The most important training session you will do is today. The next most important training session is tomorrow. And the third most important session is the one you did yesterday. What this all means is that each training session is not a stand-alone effort, but one that sits in the middle of a rolling 72-hour window that takes into account what you did yesterday, and what you're planning for tomorrow.
The famous US coach Bill Bowerman made famous the training principle of hard-easy, that is, follow-up a hard day with an easy day. A 72-hour window extends that a little, just as a rolling 3-week period would expand the context of training even further to more macro level. Training and fitness builds on itself, and builds on itself...and builds on itself even more.
The practical application of this is that you should never (or very rarely) train so hard today that you're not able to train tomorrow, or at least, not able to achieve what you want from tomorrow's training...even if it is super easy. Train, don't strain. Push yourself (hard enough), but only so hard that you could still do a bit more if you needed to. Racing is different, because that is where you do go to 100%, recognising that recovery can take some time afterwards.
As always, there are nuances - and complexities - relating to how you balance your training workload across a week(s), month(s) or year(s). And then there is periodisation of training, which is a can of worns...and where a coach can help your progression.
But the take away point is that when considering your training for today, take into account how you are feeling after your training yesterday, and how you want to feel for your planning training tomorrow. How you feel changes day by day, so reflect that in how you train so that you enjoy it most!!