First let's start with when to row harder. We all tend to want to see how hard we can pull on our first row after a long time off. Let me tell you, it will not be as hard as you want to pull. Or as hard as you can pull. After working out regularly for a period of time.
And it may be hard on the joints or muscles or organs. Sudden change can be hard on the body.
So my suggestion for when to row harder if you have not been rowing is, "Later."
Now, get out and row and do it comfortably. Do it longer. Do it each day. Do it twice a day if you cannot stick with it for a full-length workout at first. Do it strongly. But do not flail. Do not seek your limit. You are in this for the long run, not a record today.
As your muscles tighten, stretch them. As they get stronger, work longer. As you naturally want to row harder, let it happen.
And as you row more and push harder, what can you imagine or picture is happening in your body. Go to the Body of Water page for more on that.