When the body gets tried and you are fighting pain, the first technical flaw materializes through an early opening of the back and breaking of the arms. This makes the legs less capable to focus their power only on the flywheel. (The boat moves less past the oars.)
Here is how you can prepare yourself for when the going gets tough and you row yourself through H.
From the very first start stroke, and every tenth stroke, KEEP REPEATING to yourself to push the legs out from underneath the triangle of power.
This way, you can anticipate how to handle getting tired.
Keep in touch at all cost, through good and bad.