Sometimes it's important to define the "edge" in our training. By "edge" I mean that point just beyond which you think your body is capable of. The goal of training is to live on that edge without going over it.
Someone has said that the edge is easy to define but almost impossible to find. That's because the edge is different for each person. In fact, it can be different on different days even for the same person. There are days when what is normally easy is hard and what is normally hard is easy. Every athlete knows this. I experienced this today when what I thought was an "easy" exercise felt really, really hard, probably because I attempted it before warming up adequately. Sometimes the only way of knowing if you've gotten to the edge is after you've stepped over it.
Personally, I try not to push myself past the point of adaptation. And I'm always aware (or, usually aware) that the line between improvement and injury is a fine one. Athletes walk that line with caution. At the same time, I don't think we need to be afraid of training. It's just that we always have to be ready, at any moment, to face the unknown.